A firefighter sprays water out of a fire hose onto a burning house.
1. As water sprays out of the hose, the firefighter must hold on tight and be sure that his feet are firmly planted. How does the force exerted on the fireman by the spraying water depend on the velocity of the water that leaves the hose?
Answer: The force is proportional to the velocity. An increase in velocity produces an increase in force.
Why: In order to get the same amount of water to leave the hose faster it must be pushed harder by increasing the pressure in the hose. Newton's third law then requires that the water also push back harder on the fireman.
2. His feet suddenly loose traction and he begins to slide backward away from the burning house. Neglecting any air resistance as he moves back will he move at constant speed? If not, how will his speed change?
Answer: No. His speed will continue to increase as he is accelerated backward.
Why: Since the force due to sliding friction is roughly independent of speed and the force due to the spraying water is constant he will experience a constant net force which accelerates him backward.
3. The valve on the nozzle is stuck open so that he can't stop the spraying water. Is there a way he can stop himself from sliding using the spraying water? How?
Answer: Yes. He can point the hose in the direction opposite his velocity. This will decelerate him.
Why: If the hose continues to point in this direction, away from the fire, he will only stop briefly and then will start accelerating back toward the fire. When he stopped briefly he could point the hose straight up to remain stopped.
4. Not realizing how to stop himself from sliding he continues to slide backward with the water spraying toward the fire until his heels suddenly encounter a curb. When his feet suddenly stop moving he will very likely end up on his back. Why?
Answer: The force from the spraying water will produce a torque that causes him to pivot about his stopped feet, knocking him over.
Why: A torque is produced by a force that is directed perpendicular to the line connecting the pivot and the point at which this force is applied. In this case the horizontally spraying water applies a force which is perpendicular to the vertical line connecting his feet, the pivot, and his hands, where he is holding the hose.
5. If he lets go of the hose it will not continue to move straight back away from the fire, but instead will flail about wildly. Why?
Answer: The thrust provided by the spraying water is at the very end of the hose.
Why: Since the hose is flexible it can bend changing its center of mass. A constantly changing center of mass accelerated from behind by the thrust of the spraying water is a very unstable situation.
A salad spinner is a clever device designed to dry lettuce. It consists of a basket inside a bowl with a lid. The lid has a handle on top, which when rotated spins the basket and lettuce inside.
6. The crank on top turns a gear, which is connected to a gear that turns the basket. As the crank is turned slowly by hand the basket spins rapidly. How do the sizes of the gears connected to the crank and to the basket compare?
Answer: The gear connected to the crank is larger than the gear connected to the basket.
Why: For each turn of the crank and large gear, the basket and small gear turns many times causing the basket to spin faster than the crank.
7. Instead of a crank some salad spinners have a string which is pulled to spin the basket. After the string is pulled the basket keeps spinning without winding up the string. How is this accomplished?
Answer: The string must be wound around a freewheel which allows it to apply a torque when pulled, but spin freely when the pull is complete.
Why: This type of freewheel is similar to the pull started on a lawn mower.
8. Eventually the basket will stop spinning, but to stop it faster you can tip the spinner so that the basket rubs against the wall of the bowl. How does this stop the basket sooner?
Answer/Why: This provides a large force due to sliding friction that is far from the pivot point of the spinning basket, producing a torque which quickly slows the basket's spinning.