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Chapter 3 Newton’s First Law of Motion

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1 Chapter 3 Newton’s First Law of Motion
Think Questions Pages (31-41, 43) Pages 182,186 (8,42) Pages (20-31)

2 31. A space probe can be carried by a rocket into outer space
31. A space probe can be carried by a rocket into outer space. Your friend asks what kind of force keeps the probe moving after it is released from the rocket and on its own. What is your answer? Nothing keeps the probe moving. With no propelling force it continues moving in a straight line—moving of its own inertia.

3 32. In an orbiting spacecraft, you are handed two identical closed boxes, one filled with sand and the other filled with feathers. How can you tell which is which without opening the boxes? The one that is easier to shake back and forth is the one with less mass (less inertia, less resistance to changes in motion)

4 33. Many auto passengers suffer neck injuries when struck by cars from behind. How does NL of I apply? Why headrests? The body is accelerated forward with the seat, but the head remains (behind) at its current velocity until an unbalanced force pulls it forward (which is the neck, which can cause whiplash if forceful enough). Headrests provide the unbalanced force needed to accelerate the head with the body.

5 34. Tim practices a demonstration before doing it for Sunday dinner
34. Tim practices a demonstration before doing it for Sunday dinner. What concept is he illustrating, and why is he careful not to pull the tablecloth slightly upward? He is demonstrating the law of inertia (objects at rest stay at rest unless receiving an unbalanced force). Pulling upward would lift the plates and cause them to be unbalanced.

6 35. Suppose you place a ball in the middle of a wagon that is at rest and then abruptly pull the wagon forward. Describe the motion of the ball relative to the ground. Except for some change in motion due to friction between the wagon and ball, the will be no motion of the ball relative to the ground; but relative to the wagon, the ball will appear to move toward the back.

7 36. To pull a wagon across a lawn at a constant velocity, you have to exert a steady force. Does this contradict Newton’s first law? No. The key is net force. Your steady force is balancing the rolling friction, which means that the net force = 0, which means dynamic equilibrium, which means constant velocity, which means no acceleration. Removing friction would allow the wagon to continue moving at the same velocity without any more pull force.

8 Mass remains unchanged. Volume is reduced.
37. When a junked car is crushed into a compact cube, does its mass change? Its volume? Its weight? Mass remains unchanged. Volume is reduced. Weight remains unchanged (no change in position within the gravitational field so no change in gravitational force)

9 38. If an elephant was chasing you, its enormous mass would be very threatening. But if you zig-zagged, the elephant’s mass would be to your advantage. Why? Because the elephant’s mass is greater than yours, so is its inertia. Therefore, the elephant would have more difficulty switching directions than you because its inertia is greater.

10 39. When you compress a sponge, which quantity changes, mass, inertia, volume or weight?
Only volume changes.

11 40. Which has more mass, a 2-kg fluffy pillow or a 3-kg small piece of iron? More volume? Why are your answers different? The iron has more mass (greater) and the pillow has more volume (takes up more space. The answers differ because mass and volume are completely different concepts.

12 41. Is it more accurate to say that a dieting person loses mass or loses weight?
It is more accurate to say that a dieting person is losing mass (the amount of matter that composes the person). A reduction in mass leads to a reduction in weight. It is possible to change weight (only) if the person moves further from the Earth’s surface or to another place (such as the moon).

13 43. The head of a hammer is loose and you wish to tighten it by banging it against the top of a workbench. Why is it better to hold the hammer with the handle down as shown rather than with the head down? The handle stops when it hits the bench, but the relatively massive head tends to keep moving towards the handle and tightens.

14 8. When you whirl a can at the end of a string in a circular path, what is the direction of the force that acts on the can? The force is directed inward towards the axis of rotation. This inward-directed force is called centripetal force.

15 42. Can an object move along a curved path if no force acts on it?
No. An object moving along a curved path is accelerating and requires an unbalanced force to cause it to turn. This type of force is referred to as centripetal force.

16 20. Beginning from rest, a solid disk, a solid ball and a hollow disk race down an incline. What happens? The solid ball moves fastest, followed by the solid ring and the hollow disk. The solid ball has the least amount of rotational inertia because its mass is distributed closest to the center or axis of rotation.

17 31. Consider two rotating bicycle wheels, one filled with air and the other with water. Which would be more difficult to stop rotating? Explain. The bicycle wheel filled with water would be the more difficult wheel to stop rotating because it has the greater amount of rotational inertia or resistance to change its rotational motion.

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