HOW THINGS WORK
Student Questions for Spring, 1998
Louis A. Bloomfield, Professor of Physics, The University of Virginia
Think of this site as a radio call-in program that's being held on the WWW instead of the radio. If you ask how something works, using the button below, I'll try to provide an explanation. You'll find a more comprehensive discussion of many common objects in my book: How Things Work: the Physics of Everyday Life. Feel free to create links to this site; I'm trying to encourage everyone to learn about the physics and science of the world around them. Please let me know what you think about this site. -- Lou Bloomfield
March 23, 1998
Why do only certain orbitals exist in an atom?
Because the electrons in an atom move about as waves, they can follow only certain allowed orbits that we call orbitals. This limitation is equivalent to the case of a violin string--it can only vibrate at certain frequencies. If you try to make a violin string vibrate at the wrong frequency, it won't do it. That's because the string vibrates in a wave-like manner and only certain waves fit properly along the strong. Similarly, the electron in a atom "vibrates" in a wave-like manner and only certain waves fit properly around the nucleus.
When an electron hits a neon atom, does it transfer its energy to the atom and lose its own forever?
Most of the collisions between an electron and a neon atom are completely elastic--the electron bounces perfectly from the neon atom and retains essentially all of its kinetic energy. But occasionally the electron induces a structural change in the neon atom and transfers some of its energy to the neon atom. In such a case, the electron rebounds weakly and retains only a fraction of its original kinetic energy. The missing energy is left in the neon atom, which usually releases that energy as light.
Can microwave ovens leak microwaves? Is my mother's warning not to stand in front of the microwave while it's on valid?
A properly built and maintained microwave oven leaks so little microwave power that you needn't worry about it. There are also inexpensive leakage testers that you can use to see if everything is ok. It's only if you have dropped the oven or injured its door in some way that you might have cause to worry about standing near it. If it were to leak microwaves, their main effect would be to heat your tissue, so you would feel the leakage.
You said that some rooms in the physics building are made with metal to specifically keep electromagnetic waves out. How does that work?
Some experiments are so sensitive to electromagnetic waves that they must be performed inside "Faraday cages". A Faraday cage is a metal or metal screen box. Its walls conduct electricity and act as mirrors for electromagnetic waves. As long as a wave has a wavelength significantly longer than the largest hole in the walls, that wave will be reflected and will not enter the box. This reflection occurs because the wave's electric field pushes charges inside the metal walls and causes those charges to accelerate. These accelerating charges redirect (absorb and reemit) the wave in a new direction--a mirror reflection. Just as a box made of metal mirrors will keep light out, a box made with metal walls will keep electromagnetic waves out.
March 4, 1998
Is a CB radio also an AM radio?
CB or citizens band radio refers to some parts of the electromagnetic spectrum that have been set aside for public use. You can operate a CB radio without training and without serious legal constraints, although the power of your transmitted wave is strictly limited. The principle band for CB radio is around 27 MHz and I think that the transmissions use the AM audio encoding scheme. As you talk, the power of your transmission increases and decreases to represent the pressure fluctuations in your voice. The receiving CB radio detects the power fluctuations in the radio wave and moves its speaker accordingly.
March 2, 1998
What kinds of things get stored in read-only memory, as opposed to storing them on the hard drive?
When you first turn on a typical computer, it must run an initial program that sets up the operating system. This initial program has to run even before the computer is able to interact with its hard drive, so the program must be available at the very instant the computer's power becomes available. Read-only memory is used for this initial bootup operation. Unlike normal random access memory, which is usually "volatile" and loses its stored information when power is removed, read-only memory retains its information without power. When you turn on the computer, this read-only memory provides the instructions the computer uses to begin loading the operating system from the hard drive.
February 20, 1998
Why can you force the current from the n-type semiconductor to the p-type after a p-n junction has been created but you can't force current from the p-type to the n-type?
Actually, you are asking about a current of electrons, which carry a negative charge. It's true that electrons can't be sent across the p-n junction from the p-type side to the n-type side. There are several things that prevent this reverse flow of electrons. First, there is an accumulation of negative charge on the p-type side of the p-n junction and this negative charge repels any electrons that approach the junction from the p-type end. Second, any electron you add to the p-type material will enter an empty valence level. As it approaches the p-n junction, it will find itself with no empty valence levels in which to travel the last distance to the junction. It will end up widening the depletion region--the region of effectively pure semiconductor around the p-n junction; a region that doesn't conduct electricity.
Is it true that you shouldn't put a speaker near a microwave oven?
A microwave oven that's built properly and not damaged emits so little electromagnetic radiation that the speaker should never notice. The speaker might have some magnetic field leakage outside its cabinet, and that might have some effect on a microwave oven. However, most microwaves have steel cases and the steel will shield the inner workings of the microwave oven from any magnetic fields leaking from the speaker. The two devices should be independent.
February 18, 1998
Before you speak into the tape recorder, is the tape non-magnetic because half of the magnets face one way and half the other way?
Exactly. When you switch your tape recorder to the record mode, it has a special erase head that becomes active. This erase head deliberately scrambles the magnetic orientations of the tape's magnetic particles. The erase head does this by flipping the magnetizations back and forth very rapidly as the particles pass by the head, so that they are left in unpredictable orientations.
February 16, 1998
How does electric current create magnetic poles in metal? When the current goes through the metal, what makes it positive and negative?
An electric current is itself magnetic--it creates a structure in the space around it that exerts forces on any magnetic poles in that space. The magnetic field around a single straight wire forms loops around the wire--the current's magnetic field would push a magnetic pole near it around in a circle about the wire. But if you wrap the wire up into a coil, the magnetic field takes on a more familiar shape. The current-carrying coil effectively develops a north pole at one end of the coil and a south pole at the other. Which end is north depends on the direction of current flow around the loop. If current flows around the loop in the direction of the fingers of your right hand, then your thumb points to the north pole that develops at one end of the coil.
What is the difference between a magnet and an electromagnet? Why are some metals automatically magnetic?
Some metals are composed of microscopic permanent magnets, all lumped together. Such metals include iron, nickel, and cobalt. This magnetism is often masked by the fact that the tiny magnets in these metals are randomly oriented and cancel one another on a large scale. But the magnetism is revealed whenever you put one of these magnetic metals in an external magnetic field. The tiny magnets inside these metals then line up with the external field and the metal develops large scale magnetism.
However, most metals don't have any internal magnetic order at all and there is nothing to line up with an external field. Metals such as copper and aluminum have no magnetic order in them--they don't have any tiny magnets present. The only way to make aluminum or copper magnetic is to run a current through it.
February 13, 1998
Is not the current used in Europe direct current? If so, do they use transformers or do their lines get very hot? Why do our appliances not work there?
Europe uses alternating current, just as we do, however some of the characteristics of that current are slightly different. First, Europe uses 50 cycle-per-second current, meaning that current there reverses directions 100 times per second. That's somewhat slower than in the U.S., where current reverses 120 times per second. Second, their standard voltage is 240 volts, rather than the 120 volts used in the U.S.
While some of our appliances won't work in Europe because of the change in cycles-per-second, the biggest problem is with the increase in voltage. The charges entering a U.S. appliance in Europe carry about twice the energy per change (i.e. twice the voltage) and this increased "pressure" causes about twice the number of charges per second (i.e. twice the current) to flow through the appliance. With twice the current flowing through the appliance and twice as much voltage being lost by this current as it flows through the appliance, the appliance is receiving about four times its intended power. It will probably burn up.
February 11, 1998
If only electrons move around, why do you keep using positive charges in the demos?
It's useful to describe moving electric charges as a current and for that current to flow in the direction that the charges are moving. Suppose that we define current as flowing in the direction that electrons take and look at the result of letting this current of electrons flow into a charge storage device. We would find that as this current flowed into the storage device, the amount of charge (i.e. positive) charge in that device would decrease! How awkward! You're "pouring" something into a container and the contents of that container is decreasing! So we define current as pointing in the direction of positive charge movement or in the direction opposite negative charge movement. That way, as current flows into a storage device, the charge in that device increases!
How come the flashlight works when you switch the batteries but my walkman or gameboy doesn't?
The bulb in a battery doesn't care which way current flows through it. The metal has no asymmetry that would treat left-moving charges differently from right-moving charges. That's not true of the transistors in a walkman or gameboy. They contain specialized pieces of semiconductor that will only allow positive charges to move in one direction, not the other. When you put the batteries in backward and try to propel current backward through its parts, the current won't flow and nothing happens.
Why are batteries so expensive?
They contain highly purified and refined chemicals and are actually marvels of engineering. It's more surprising to me that they are so cheap, given how complicated they are to make.
How are you "shocked"?
Your body is similar to salt water and is thus a reasonably good conductor of electricity. Once current penetrates your skin (which is insulating), it flows easily through you. At high currents, this electricity can deposit enough energy in you to cause heating and thermal damage. But at lower currents, it can interfere with normal electrochemical and neural process so that your muscles and nerves don't work right. It takes about 0.030 amperes of current to cause serious problems for your heart, so that currents of that size can be fatal.
February 9, 1998
Will the first battery run out before the next because it pumps out all its charges to the other?
No, each battery in a chain takes positive charge at its negative terminal and pumps that charge to its positive terminal. In doing so, it uses up its chemical potential energy. The amount of energy consumed is exactly proportional to the amount of charge pumped. Since each battery pumps the same amount of charge, the batteries are consumed at the same rate.
How do rechargeable batteries get recharged?
You can recharge any battery by pushing charge through it backward (pushing positive charge from its positive terminal to its negative terminal). However, some batteries don't take this charge well or heat up. The ones that recharge most effectively are those that can rebuild their chemical structures most effectively as they operate backward.
What is the reason for the coiled wire in a battery circuit?
The coiled wire represents the filament of the light bulb--the portion of the circuit that is supposed to receive the electric power flowing from the batteries.
If the battery separates charges even while it's off, how come it doesn't light up when it's off?
The battery stops separating charges once enough have accumulated on its terminals. If the flashlight is off, so that charges build up, then the battery soon stops separating charge and the light bulb doesn't light.
February 6, 1998
What keeps the earth stable so that it doesn't get pulled up into the "magnet"?
If you are asking why doesn't the earth itself get pulled up toward a large magnet or electromagnet that I'm holding in my hand, the answer is that the magnetic forces just aren't strong enough to pull the magnet and earth together. I'm holding the two apart with other forces and preventing them from pulling together. The forces between poles diminish with distance. Those forces are proportional to the inverse square of the distance between poles, so they fall off very quickly as the poles move apart. Moreover, each north pole is connected to a south pole on the same magnet, so the attraction between opposite poles on two separate magnets is mitigated by the repulsions of the other poles on those same magnets. As a result, the forces between two bar magnets fall over even faster than the simple inverse square law predicts. It would take an incredible magnet, something like a spinning neutron star, to exert magnet forces strong enough to damage the earth. But then a neutron star would exert gravitational forces that would damage the earth, too, so you'd hardly notice the magnetic effects.
Is the earth a huge magnet? If so, how does it do this without being made out of metal?
The earth is a huge magnet and it is made out of metal. The earth's core is mostly iron and nickel, both of which can be magnetic metals. However, the earth's magnetism doesn't appear to come from the metal itself. Current theories attribute the earth's magnetism to movements in and around the core. There are either electric currents associated with this movement or some effects that orient the local magnetization of the metal. I don't think that there is any general consensus on the matter.
February 4, 1998
Why do poles have to come in pairs?
There don't appear to be isolated poles in our universe, or at least none have been found. That's just the way it is. As a result of this situation, the only way to create magnetism is through its relationship with electricity. When you use electricity to create magnetic fields, you effectively create equal pairs of poles--as much north pole as south pole.
If the train track gets bumpier in effect with increasing speed, why is it that your car bumps less when you go over a speed bump fast instead of slow?
Actually, if you drive fast over a real speed bump, it's not good for your wheels and suspension. The springs in your car do protect the car from some of the effects of the bump, but not all of them. However, imagine driving over a speed bump on a traditional bicycle--one that has no spring suspension. The fast you drive over that bump, the more it will throw you into the air.
Is the red light effect in xerographic copiers the same concept behind red lights in a dark-room? Does film have the same sort of properties?
Yes. The light sensitive particles in black-and-white film don't respond to red light because the energy in a photon of red light doesn't have enough energy to cause the required chemical change. In effect, electrons are being asked to shift between levels when the light hits them and red light can't make that happen in the film.
Are black lights less or more conducive to charging the particles in film?
They are generally more conducive. Black light is actually ultraviolet light and its photons carry more energy than any visible photon. They can cause chemical changes in many materials, including skin.
Does this photoconductor stuff have to do with why you can only develop film in the dark?
Yes. Particles of light, photons, cause chemical changes in the film. You can work with some black-and-white films in red light because red light photons don't have enough energy to cause changes in those films. However, color film and some black-and-white films require complete darkness during processing. If you expose them to any visible light, you'll cause chemistry to occur.
Are all metals magnetically charged?
First, magnets don't involve charges, they involve poles. So the question should probably be "are all metals magnetically poled?" The answer to this question is that they are never poled--they never have a net pole. They always have an even balance of north and south pole. However, there are some metals that have their north and south poles separated from one another. A magnetized piece of steel is that way. Only a few metals can support such separated poles and we will study those metals in a few weeks.
How do shampoo and conditioners in one work if shampoos have negative charges on one side and conditioners have positive charges on one side?
I don't know. That question has puzzled me for years. The mixture should find its molecules clinging together. They must contain something that keeps the oppositely charged systems separate from one another so that they don't aggregate.
How do color copiers work?
They assemble 4 colors, yellow, cyan, magenta, and black together to form the final image. The photoconductor creates charge images using blue, red, green, and white illumination successively and uses those images to form patterns of yellow, cyan, magenta, and black toner particles. These particles are then superimposed to form the final image, which appears full color. Naturally, the photoconductor used in such a complicated machine must be sensitive to the whole visible spectrum of light.
Would placing a blue filter on a xerox machine prevent it from making copies, since blue light has more energy than red?
No. Blue light causes the photoconductor to conduct. When you use white light in a xerographic copier, it's the blue and green portions of the light that usually do the copying. The red is wasted.
Is it physically possible for a baseball player to hit a baseball that has been pitched 60 ft away at 90-95 mph? If so, why are the highest baseball records between 3 and 4 out of ten?
If the ball was pitched straight and true, the same way every pitch, good batters could hit every one. There is enough time in the wind-up and pitch for the batter to determine where and when to swing and to hit the ball just right. But the pitches vary and the balls curve. That limits the batter's ability to predict where the ball is going. There aren't any physical laws that limit a batter's ability to hit every ball well, but there are physiological and mental limits that lower everyone's batting average.
Why is it that you're supposed to unplug your trailer lights before backing your boat down the boat ramp and into the water? It seems that less corrosion/rust forms when no electricity is run through them when they're under water.
The main reason for disconnecting trailer lights is so that they aren't hot when they enter the water. Because of the uneven stresses that occur during a sudden, non-uniform temperature change, hot glass can shatter when you dip it suddenly in cold water. Even if the bulbs survive, when you immerse an electric circuit in water, you create all sorts of extra paths for electricity. Some of these paths just waste energy as heat, but others cause chemical reactions to occur--similar to the chemical reactions in a battery. To avoid these problems, it's best to remove the power from your lights before you immerse them.
February 2, 1998
Why don't the negatively charged particles push toward the positively charged particles? I thought opposites attract?
You're right, they do. If I implied that negative charges aren't pulled toward positive charges, I was wrong.
Can the electric current be taken out of the metal where the charge will not carry?
While charges can move freely through a metal, allowing the metal to carry electric current, it's much harder for charges to travel outside of a conductor. Charges can move through the air or through plastic or glass, but not very easily. It takes energy to pull the charges out of a metal and allow them to move through a non-metal. Most of the time, this energy requirement prevents charges from moving through insulators such as plastic, glass, air, and even empty space.
How does one "pull up their legs"? Wouldn't you have to jump in some way or another?
It is possible to simply pull up your legs. When you do that, you reduce the downward force your feet exert on the ground and the ground responds by pushing upward on your feet less strongly. With less upward force to support you, you begin to fall.
Is the spin up and spin down that you were talking about the 4th quantum number?
By some counting schemes, the answer is yes. In an atom, each electron has three spatial quantum numbers and one intrinsic quantum number--spin.
Don't you need to follow Hund's rule when filling the levels with electrons?
Hund's rules apply to atomic and molecular systems, but related rules do lead to magnetism in some solids such as iron and nickel.
If electrons can't change levels, how can a photoconductor help them change one level to another?
In a metal, electrons can easily shift from one level to another empty level because the levels are close together in energy. In a full insulator, it's very difficult for the electrons to shift from one level to an empty level because all of the empty levels are far above the filled levels in energy. In a photoconductor, the empty levels are modestly above the filled levels in energy, so a modest amount of energy is all that's needed to shift an electron. This energy can be supplied by a particle or "photon" of light. An illuminated photoconductor conducts electricity.
How do dryer sheets diminish the clothes' static?
They leave a layer of conditioning soap on the clothes and this soap attracts moisture. The moisture conducts electricity just enough to allow static charge to dissipate.
How do you get static out of hair?
If you put a conditioner on your hair, it will attract enough moisture to allow static charge to dissipate.
Does an MRI work in the same way as a copier (or puts you in a magnetic field and copies an image of your body)?
No, an MRI uses a very different technique for imaging your body. A copier uses light to examine the original document while an MRI machine uses the magnetic responses of hydrogen atoms to map your body.
How does one create an electric or magnetic field?
The simplest way to make these fields is with electric charges (for an electric field) or with magnets (for a magnetic field). Charges are naturally surrounded by electric fields and magnets are naturally surrounded by magnetic fields. But fields themselves can create other fields by changing with time. That's how the fields in a light wave work--the electric field in the light wave changes with time and creates the magnetic field and the magnetic field changes with time and creates the electric field. This team of fields can travel through space without any charge or magnets nearby.
January 28, 1998
Does the spinning wheel slow down when flipped and cause you to spin--that is, now you have momentum you didn't have before and didn't it come from the wheel?
Flipping the spinning wheel completely over is equivalent to stopping it from turning and then making it turn the opposite direction. During the flip, things are (unfortunately) complicated, but once the flip is complete everything is simple. You can imagine that I just stopped the wheel from turning one direction and started it turning in the opposite direction.
From that point of view, it's not too hard to see that I should end up spinning in the direction that the wheel was originally spinning in, and that the wheel should end up spinning in the opposite direction. Overall, my angular momentum did come from the wheel. The wheel gave up that original angular momentum and then it gave up even more as it began to spin backwards.
Is hydroplaning a form of sliding friction?
Yes. When you begin to skid on dry pavement, your car's wheels are experience sliding friction. The sliding friction force on those wheels is pretty strong because rubber and asphalt grip one another quite well. But if there is a layer of water in between the two surfaces, the water lubricates them and the force of sliding friction drops precipitously. That's why you can skid so far when you are hydroplaning on a thin layer of water.
January 26, 1998
If ball bearings create no friction, why do bearings have bearing grease as an essential ingredient?
Actually, some bearings are dry (no grease or oil) and still last a very long time. The problem is that the idea touch-and-release behavior is hard to achieve in a bearing. The balls or rollers actually slip a tiny bit as they rotate and they may rub against the sides or retainers in the bearing. This rubbing produces wear as well as wasting energy. To reduce this wear and sliding friction, most bearings are lubricated.
How do anti-lock brake systems work?
If you brake your car too rapidly, the force of static friction between the wheels and the ground will become so large that it will exceed its limit and the wheels will begin to skid across the ground. Once skidding occurs, the stopping force becomes sliding friction instead of static friction. The sliding friction force is generally weaker than the maximum static friction force, so the stopping rate drops. But more importantly, you lose steering when the wheels skid. An anti-lock braking system senses when the wheels suddenly stop turning during braking and briefly release the brakes. The wheel can then turn again and static friction can reappear between the wheel and the ground.
If you walk up 10 steps, one by one, do you exert the same amount of energy if you walk up the same set of 10 steps two by two? How are energy and effort related, or are they?
Ideally, it doesn't matter how many steps you take with each step--the work you do in lifting yourself up a staircase depends only on your starting height and your ending height (assuming that you don't accelerate or deccelerate in the overall process and thus change your kinetic energy, too). But there are inefficiencies in your walking process that lead you to waste energy as heat in your own body. So the energy you convert from food energy to gravitational potential energy in climbing the stairs is fixed, but the energy you use in carrying out this procedure depends on how you do it. The extra energy you use mostly ends up as thermal energy, but some may end up as sound or chemical changes in the staircase, etc.
January 23, 1998
How can a ball create thermal energy or "get hotter"?
When a ball bounces, some of its molecules slide across one another rather than simply stretching or bending. This sliding leads to a form of internal sliding friction and sliding friction converts useful energy into thermal energy. The more sliding friction that occurs within the ball, the less the ball stores energy for the rebound and the worse the ball's bounce. The missing energy becomes thermal energy in the ball and the ball's temperature increases.
January 21, 1998
You discussed how an egg doesn't bounce because it doesn't have time and instead it breaks. Why, then, does a mouse ball (in a computer mouse) or a bowling ball not bounce? It doesn't break, so why doesn't the support force make it bounce back upward. Does this relate to elasticity?
Actually, both a mouse ball and a bowling ball will bounce somewhat if you drop them on a suitably hard surface. It does have to do with elasticity. During the impact, the ball's surface dents and the force that dents the ball does work on the ball--the force on the ball's surface is inward and the ball's surface moves inward. Energy is thus being invested in the ball's surface. What the ball does with this energy depends on the ball. If the ball is an egg, the denting shatters the egg and the energy is wasted in the process of scrambling the egg's innards. But in virtually any normal ball, some or most of the work done on the ball's surface is stored in the elastic forces within the ball--this elastic potential energy, like all potential energies, is stored in forces. This stored energy allows the surface to undent and do work on other things in the process. During the rebound, the ball's surface undents. Although it's a little tricky to follow the exact flow of energy during the rebound, the elastic potential energy in the dented ball becomes kinetic energy in the rebounding ball. But even the best balls waste some of the energy involved in denting their surfaces. That's why balls never bounce perfectly and never return to their original heights when dropped on a hard, stationary surface. Some balls are better than others at storing and returning this energy, so they bounce better than others.
When an egg falls and hits the table, the table pushes up on it, doesn't it? The same with a bouncing ball?
Yes, when a falling object hits a table, the table pushes up on the falling object. What happens from then on depends on the object's characteristics. The egg shatters as the table pushes on it and the ball bounces back upward.
In Exercise 7 from Section 1.1 of the book (yanking a sheet on a pad of paper), the answer the book gives deals with the paper pad's inertia, but is it also correct that the edge of the paper (where it's attached to the pad) can only stand a certain amount of force? Small forces allow it to remain attached, large forces cause it to rip?
Yes, you are exactly right. If you pull gently on the paper, it will drag the pad with it. But if you pull hard, the paper will tear away from the pad. So while inertia makes it difficult for the pad to follow the sheet of paper, it's ultimately the amount of force the paper exerts on the pad that determines whether those two stay together or separate.
I don't understand work done without any acceleration. Since F=ma and a=0, F=0 and thus W=0.
You are merging two equations out of context. The force you exert on an object can be non-zero without causing that object to accelerate. For example, if someone else is pushing back on the object, the object may not accelerate. If the object moves away from you as you push on it, then you'll be doing work on the object even though it's not accelerating. The only context in which you can merge those two equations (Force=mass x acceleration and Work=Force x distance) is when you are exerting the only force on the object. In that case, your force is the one determine the object's acceleration and your force is the one involved in doing work. In that special case, if the object doesn't accelerate, then you do no work because you exert no force on the object! If someone else is pushing the object, then the force causing it to accelerate is the net force and not just your force on the object. As you can see, there are many forces around and you have to be careful tacking formulae together without thinking carefully about the context in which they exist.
When a rubber ball bounces or rebounds, does the weight of the ball determine how many times it bounces?
Each time the ball bounces, it rises to a height that is a certain fraction of its height before that bounce. The ratio of these two heights is the fraction of the ball's energy that is stored and returned during the bounce. A very elastic ball will return about 90% of its energy after a bounce, returning to 90% of its original height after a bounce. A relatively non-elastic ball may only return about 20% of its energy and bounce to only 20% of its original height. It is this energy efficiency that determines how many times a ball bounces. The missing energy is usually converted into thermal energy within the ball's internal structure.
What is thermal energy?
While we ordinarily associate energy with an object's overall movement or position or shape, the individual atoms and molecules within the object can also have their own separate portions of energy. Thermal energy is the energy associated with the motions and positions of the individual atoms within the object. While an object may be sitting still, its atoms and molecules are always jittering about, so they have kinetic energies. When they push against one another during a bounce, they also have potential energies. These internal energies, while hard to see, are thermal energy.
January 19, 1998
How does the egg (sitting on a table) hold up the table? If the "weight vs. support force of table" is not always an equal pair then how is the "support force of the egg vs. the table" an equal pair?
When an egg is sitting on a table, each object is exerting a support force on the other object. Those two support forces are equal in magnitude (amount) but opposite in direction. To be specific, the table is pushing upward on the egg with a support force and the egg is pushing downward on the table with a support force. Both forces have the same magnitude--both are equal in magnitude to the egg's weight. The fact that the egg is pushing downward on the table with a "support" force shows that not all support forces actually "support" the object they are exert on. The egg isn't supporting the table at all. But a name is a name and on many occasions, support forces do support the objects they're exerted on.
If there is an upward force on the egg when it hits the table, why doesn't it bounce upward?
The enormous upward force on the egg when it hits the table does cause the egg to accelerate upward briefly. The egg loses all of its downward velocity during this upward acceleration. But the egg breaks before it has a chance to acquire any upward velocity and, having broken, it wastes all of its energy ripping itself apart into a mess. If the egg had survived the impact and stored its energy, it probably would have bounced, at least a little. But the upward force from the table diminished abruptly when the egg broke and the egg never began to head upward for a real bounce.
What effects do forces acting on an object which are not of the same pair have on one another? i.e. the force pulling the egg downward and the potential force of the table? Are they equal upon impact and there a pair?
Different forces acting on a single object are not official pairs; not the pairs associated with Newton's third law of action-reaction. While it is possible for an object to experience two different forces that happen to be exactly equal in magnitude (amount) but opposite in direction, that doesn't have to be the case. When an egg falls and hits a table, the egg's downward weight and the table's upward support force on the egg are equal in magnitude only for a fleeting instant during the collision. That's because the table's support force starts at zero while the egg is falling and then increases rapidly as the egg begins to push against the table's surface. For just in instant the table pushes upward on the egg with a force equal in magnitude to the egg's weight. But the upward support force continues to increase in strength and eventually pushes a hole in the egg's bottom.
Could you explain next time why ramps make it easier to move objects
Yes, I'll do that.
January 16, 1998
When you drop a small rubber ball and a large rubber ball simultaneously, why do they both hit the floor at the same time?
The fact that both balls fall together is the result of a remarkable balancing effect. Although the larger ball is more massive than the smaller ball, making the larger ball harder to start or stop, the larger ball is also heavier than the smaller ball, meaning that gravity pulls downward more on the larger ball. The larger ball's greater weight exactly compensates for its greater mass, so that it is able to keep up with the smaller ball as the two objects fall to the ground. In the absence of air resistance, the two balls will move exactly together-the larger ball with its greater mass and greater weight will keep up with the smaller ball.
When you throw a ball upward and claim that there is no upward force on it as it rises, why don't you count your hand? The ball was thrown up, so there was an upward force on it! I'm confused.
While you are throwing the ball upward, you are pushing it upward and there is an upward force on the ball. But as soon as the ball leaves your hand, that upward force vanishes and the ball travels upward due to its inertia alone. In the discussion of that upward flight, I always said "after the ball leaves your hand," to exclude the time when you are pushing upward on the ball. Starting and stopping demonstrations are often tricky and I meant you to pay attention only to the period when the ball was in free fall.
How does the floor exert a force?
When you stand on the floor, the floor exerts two different kinds of forces on you--an upward support force that balances your downward weight and horizontal frictional forces that prevent you from sliding across the floor. Ultimately, both forces involve electromagnetic forces between the charged particles in the floor and the charged particles in your feet. The support force develops as the atoms in the floor act to prevent the atoms in your feet from overlapping with them. The frictional forces have a similar origin, although they involve microscopic structure in the surfaces.
Why is there more gravity acting on larger, more massive objects?
The fact that more massive objects also weigh more is just an observation of how the universe works. However, any other behavior would lead to some weird consequences. Suppose, for example, that an object's weight didn't depend on its mass, that all objects had the same weight. Then two separate balls would each weigh this standard amount. But now suppose that you glued the two balls together. If you think of them as two separate balls that are now attached, they should weigh twice the standard amount. But if you think of them as one oddly shaped object, they should weigh just the standard amount. Something wouldn't be right. So the fact that weight is proportional to mass is a sensible situation and also the way the universe actually works.
Why does the bigger ball have more gravity pulling on it? Because it weighs more? Which causes which?
The force that gravity exerts on an object is that object's weight. An object that has more gravity pulling on it weighs more and vice versa.
How is there inertia on earth? I though that inertia was just in space.
Inertia is everywhere. Left to itself, an object will obey inertia and travel at constant velocity. In deep space, far from any planet or star that exerts significant gravity, an object will exhibit this inertial motion. But on earth, the earth's gravity introduces complications that make it harder to observe inertial motion. A ball that's thrown up in the air still exhibits inertial effects, but its downward weight prevents the ball from following its inertia alone. Instead, the ball gradually loses its upward speed and eventually begins to descend instead. So inertia is the basic underlying principle of motion while gravity is a complicating factor.
When you drop a baseball and a bowling ball, you say that its velocity acts faster and faster as it falls. How can you say that the acceleration is constant at 9.8 m/s2? If it is falling faster and faster wouldn't the acceleration change also until the object reaches terminal velocity and then it would be accelerating at 9.8 m/s2?
It's very important to distinguish velocity from acceleration. Acceleration is caused only by forces, so while a ball is falling freely it is accelerating according to gravity alone. In that case it accelerates downward at 9.8 m/s2 throughout its fall (neglecting air resistance). But while the ball's acceleration is constant, its velocity isn't. Instead, the ball's velocity gradually increases in the downward direction, which is to say that the ball accelerates in the downward direction. Velocity doesn't "act"--only forces "act." Instead, a ball's velocity shifts more and more toward the downward direction as it falls.
About terminal velocity: when an object descends very rapidly through the air, it experiences a large upward force of air resistance. This new upward force becomes stronger as the downward speed of the object becomes greater. Eventually this upward air resistance force balances the object's downward weight and the object stops accelerating downward. It then descends at a constant velocity--obeying its inertia alone. This special downward speed is known as "terminal velocity." An object's terminal velocity depends on the strength of gravity, the shape and other characteristics of the object, and the density and other characteristics of the air.
Why is it that when people jump, they don't bounce up?
A ball bounces because its surface is elastic and it stores energy during the brief period of collision when the ball and floor are pushing very hard against one another. Much of this stored energy is released in a rebound that tosses the ball back upward for another bounce. But people don't store energy well during a collision and they don't rebound much. The energy that we should store is instead converted into thermal energy--we get hot rather than bouncing back upward.
When people are able to bend spoons or move tables with their minds (if this is actually possible and not just a hoax), what sort of force is being exerted on the object? Is it possible to create forces with the mind?
I'm afraid that spoon bending is simply a hoax. While there are electrochemical processes going on in the mind that exert detectable forces on special probes located outside the head, these forces are so small that they are incapable of doing anything as demanding as bending a spoon. Spoon bending and all other forms of telekinesis are simply tricks played on gullible audiences.
I'm confused by the "weird world" in which the baseball would reach the ground before the bowling ball.
In my "weird world," I asserted that gravity exerted the same force on each object, regardless of that object's mass. In that weird world, everything would have the same weight. When you dropped objects in that world, the less massive ones would accelerate faster (same weight but less mass means larger acceleration) and would hit the floor faster. But this weird world is filled with contradictions--two people who held hands would become a single object and would suddenly lose half their total weight. As a result, two people who fell separately would accelerate downward faster than two identical people who held hands and became a single object. This would be a weird world indeed. If this discussion still leaves you confused, forget the weird world completely. In our real world, weight is proportional to mass so everything falls together. Everything is beautifully consistent--a central feature of physics and one of the reasons physics appeals to people with a taste for logic and order. Despite its complexity, the real universe is remarkably ordered and follows an elegant set of rules with absolute precision.
Questions from September, 1996
Questions Asked by Students at the University of Virginia, Spring, 1997
Information about the Book: "How Things Work"
Resources for "How Things Work"
Institutions Teaching "How Things Work"
Course Materials for Physics 105 at the University of Virginia
Course Materials for Physics 106 at the University of Virginia