One way of measuring a person's body fat content is by "weighing" them under water. This works because fat tends to float on water, it is less dense than water, while muscle and bone tend to sink, they are more dense. Knowing your "weight" under water as well as your real weight out of water, the percentage of your body's volume that is made up of fat can easily be estimated. This is only an estimate since it assumes that your body is made up of only two substances, fat (low density) and everything else (high density). Compare the "weight" measured by an underwater scale under the following circumstances. Quotes are placed around weight to indicate that the measurement read on the scale is not your true weight, the force applied to your body by gravity, but a measurement of the net downward force on the scale.
1. If a large percentage of your body's volume were fat compared to a small percentage?
2. If you were fully immersed in the water compared to being partially immersed?
3. If you were fully immersed at the shallow end of a pool compared to being fully immersed at the deep end?
4. If your lungs were full of air compared to when all of the air has been blown out of your lungs?
5. Salt water is more dense than fresh water. Would you "weigh" more if you were fully immersed in salt water or fresh water?
The brakes on most cars make use of a hydraulic system to transfer the force applied by your foot at the brake pedal to the brake pads at the car's wheels, which slow you down. The hydraulic system consists of a fluid filled tube connected at both ends to pistons. The piston at one end is attached to the brake pedal and the piston at the other end is attached to the brake pad. When the brakes are applied the brake pad, which is fixed relative to the car's body, pushes against a disk (the rotor) that spins with the wheel. The sliding friction between the brake pad and the disk slows the car down.
6. Why is the fluid used to fill such a hydraulic system a liquid rather than a gas?
7. When you push on the brake pedal what happens to the pressure of the hydraulic fluid?
8. The sliding friction between the brake pad and the disk slows the car down. Where does the car's kinetic energy go in this process? Assume you are driving on a flat horizontal stretch of road.