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Instructor: Dr. Scott A. Yost Office: 307 Nielsen Physics Building Hours: Monday, Wednesday and Friday, 1-2 PM or by appointment Textbook: L.A. Bloomfield, How Things Work, 2nd ed. |
Phone: 974-7852 E-Mail: syost@utk.edu Web Site: http://homework.phys.utk.edu/phys101 |
These are the answers to the even-numbered exercises and problems, and all of the cases assigned in Chapter 5. The answers to the others appear in the back of the textbook.
Exercises
4. At higher temperatures, the molecules in maple syrup have more thermal energy and are able to break free of one another more easily as the syrup flows.10. When the valve abruptly closes, the moving water in the pipe must suddenly get rid of its momentum and it shakes the pipes as it does. When the valve opens, the water can slowly pick up speed and momentum, so no shaking occurs.
14. The fast-moving stick creates a noisy turbulent wake.
22. Pressure drag is pushing the antenna backward.
24. The spokes experience pressure drag because they produce turbulent wakes as the wheel spins.
56. If a spacecraft were to go faster, its increased kinetic energy would cause it to swing away from the earth's surface into a huge elliptical orbit. It would then take longer to complete the orbit, not shorter.
Problems
2. Your blood pressure would have to increase by about 23% to compensate.
10. You would weigh half your normal weight if you were about 2640 km above the earth's surface. (Remember that the earth's radius is 6378 km.)
12. You are exerting gravitational forces of 3.27 x 10-9 Newtons on one another.
Cases
1a. gravity, tension (rope), lift, and (pressure) drag.
1b. The forward component of velocity decreases.
1c. The lift force tilts forward, and the drag tilts upward. Gravity is unchanged.
1d. If the forward component of the lift balances the rearward component of the drag, a constant horizontal velocity is maintained.
1e. The upward force from the rising air can balance gravity.3a. Because the parachute slows the air down, increasing its pressure.
3b. There is a turbulent wake above the parachute, with average atmospheric pressure.
3c. The upward force is due to pressure drag from the increased pressure below the parachute, relative to above it.
3d. If you fall faster, the air slows down more upon approaching the bottom of the parachute, and correspondingly, the pressure increases more.
3e. The net force is upward when you decelerate, and the parachute's drag must be greater than your weight.
3f. The parachute's drag is equal to your weight in this case.
3g. You feel weightless at first, then heavier than normal when the parachute first catches you. When decending at constant speed, you feel your normal weight.
4a. The pressure is less on the outside of the sail, where it is moving faster to bend around the curve of the sail.
4b. The pressure inside the sail is greater than normal, because the sail slows the wind down as it catches and deflects it.
4c. The overall force is toward the outside of the sail.
4d. The wind is blowing the boat to the east. If the keel is angled toward the northeast, the boat will move to the northeast due to a component of the force of the water on the keel in reaction to the wind. (It helps to draw a picture.)
4e. In addition to lift, the wind also always produces drag, which forces the boat downwind unless there is a keel to counteract this by pressing on the water.
4f. There is also drag on the boat due to motion of the water around the boat. This drag increases with speed until the boat reaches a constant speed.
4g. When the rudder deflects water to the right, there is an opposite force on the rudder which deflects it to the left. This then generates a clockwise torque on the boat, assuming the rudder is in back of the boat.
6a. The crew members would fly abruptly to the right.
6b. The projectile must have had a lot of momentum, since the creature was propelled backwards by its collision. The same momentum should have been transferred to the person shooting the weapon when it fired, so he should have been thrown backward as well.
6c. Spacecraft cannot use aerodynamic forces to turn, as aircraft do near the ground. They can only turn by firing their rocket in a direction which turns the spacecraft. Firing the rocket straight backwards can't do this. They need to fire it to the side.
6d. Rockets rely on ejecting a large fraction of their weight out the back in order to gain speed needed to leave a planet. On an earth-like planet, this normally requires carrying an entire stage full of fuel, and ejecting it after liftoff.
6e. Her impact force does not depend on what she hits. She loses just as much momentum hitting a spacecraft as she would hitting the ground, so falling onto the spacecraft is just as fatal as falling onto the ground.
Department of Physics University of Tennessee