Week 8
distorted easily.
26. Some amusement park rides move you back and forth in a horizontal direction. Why is this motion so much more disturbing
to your body than cruising at a high speed in a jet airplane?
E.26 You only feel accelerations, so rapid side-to-side motion is easily felt. In contrast, rapid travel at constant velocity
involves no acceleration and produces no sensations at all.
27. You are traveling in a subway along a straight, level track at a constant velocity. If you close your eyes, you can’t tell which
way you’re heading. Why not?
E.27 While you can feel accelerations, you can’t feel velocity.
28. Moving a can of spray paint rapidly in one direction will not mix it nearly as well as shaking it back and forth. Why is it so
important to change directions as you mix the paint?
E.28 To make the paint move relative to the can and mix as a result, you must make the can accelerate. The paint will then
coast around inside the can and slam into the walls. If you just move the can quickly in one direction, the paint and
can will coast along together and there will be little mixing.
29. Why does a baby’s rattle only make noise when the baby moves it back and forth and not when the baby moves it steadily in
one direction?
E.29 When the rattle accelerates, the beads inside it continue on and hit the walls of the rattle. The rattle then makes noise.
30. In some roller coasters, the cars travel through a smooth tube that bends left and right in a series of complicated turns. Why
does the car always roll up the right-hand wall of the tube during a sharp left-hand turn?
E.30 When the track makes a sharp left-hand turn, the car needs a strong leftward force to follow it. The car obtains that
leftward force by riding up on the right-hand wall of the tube.
31. Railroad tracks must make only gradual curves to prevent trains from derailing at high speeds. Why is a train likely to derail
if it encounters a sharp turn while it’s traveling fast?
E.31 The sharper the curve, the more centripetal force the train needs to accelerate around the curve. If the track can’t
supply it . . . disaster.
32. Police sometimes use metal battering rams to knock down doors. They hold the ram in their hands and swing it into a door
from about 1 m away. How does the battering ram increase the amount of force the police can exert on the door?
E.32 The police can use a long period of time and a modest forward force to give the battering ram forward momentum.
The battering ram can then transfer this momentum to a door with a huge force exerted for a short period of time.
33. When a moving hammer hits a nail, it exerts the enormous force needed to push the nail into wood. This force is far greater
than the hammer’s weight. How is it produced?
E.33 The nail exerts an enormous force on the hammer to slow it down. The hammer pushes back, driving the nail into the
wood.
34. A hammer’s weight is downward, so how can a hammer push a nail upward into the ceiling?
E.34 When the moving hammer encounters the nail, the nail pushes back extremely hard on the hammer to stop the
hammer from penetrating into the nail. The hammer pushes back on the nail, propelling it into the ceiling. While
gravity also pushes on the hammer and slows its upward motion, the weight forces in this situation are trivial
compared to the forces between hammer and nail.
35. As you swing back and forth on a playground swing, your apparent weight changes. At what point do you feel the heaviest?
E.35 At the bottom of each swing.
36. Some stores have coin-operated toy cars that jiggle back and forth on a fixed base. Why can’t these cars give you the feeling
of actually driving in a drag race?
E.36 A jiggling car can only accelerate you forward for a brief period of time, while a true drag racer will accelerate you
forward for a long period of time. You can feel the forward acceleration as a backward gravity-like sensation.
37. A salad spinner is a rotating basket that dries salad after washing. How does the spinner extract the water?
E.37 As the salad undergoes rapid centripetal acceleration, the water travels in straight lines and runs off the salad.
38. People falling from a high diving board feel weightless. Has gravity stopped exerting a force on them? If not, why don’t they
feel it?
E.38 A falling person's internal organs all fall together without having to support one another. The absence of internal
forces within a person's body is what leads them to believe that they are weightless.
39. When your car travels rapidly over a bump in the road, you suddenly feel weightless. Explain.
E.39 After going over a bump, the car begins to accelerate downward and your apparent weight is briefly less than your
real weight.
40. Astronauts learn to tolerate weightlessness by riding in an airplane (nicknamed the “vomit comet”) that follows an unusual
trajectory. How does the pilot direct the plane in order to make its occupants feel weightless?
E.40 The pilot steers the "vomit comet" in the path of a falling object: a parabolic arc through the air. The net force on the
plane is then exactly equal to its weight and it is truly falling. The objects inside it fall as well and everyone inside
feels weightless.
41. You board an elevator with a large briefcase in your hand. Why does that briefcase suddenly feel particularly heavy when the
elevator begins to move upward?
E.41 As the car accelerates upward, you must pull upward on the briefcase extra hard to make it accelerate upward too.
42. As your car reaches the top in a smoothly turning Ferris wheel, which way are you accelerating?
E.42 You are accelerating downward at the top of a smoothly turning Ferris wheel.
E.42 In uniform circular motion, you always accelerate toward the center of the circle.
Additional Questions:
1. What is centripetal acceleration?
a. Acceleration directed towards the center of a circle.
2. The author of the textbook speaks of the “feeling” of acceleration. How does this differ from
actual acceleration?
a. The feeling of acceleration is always opposition the direction of the actual acceleration.
3. What is apparent weight?
a. The combined effect of weight and feeling of acceleration.