1. The centers of gravity of three trucks parked on a hill are shown by the dots. Which truck(s) will tip over?
Ch 8-2
1. A 2. B 3. C

2. The centers of gravity of three trucks parked on a hill are shown by the dots. Which truck(s) will tip over?
Ch 8-2
Answer: 1
The center of gravity of Truck A is not above an area of support; the centers of gravity of Trucks B and C are above areas of support. Therefore only Truck A will tip over.
1. A 2. B 3. C

3. When she shakes the basketful of berries, the larger berries will
Ch 8-3
1. sink to the bottom. 2. go to the top. 3. not particularly sink or rise, but like the smaller berries, be randomly distributed.

4. When she shakes the basketful of berries, the larger berries will
Ch 8-3
Answer: 2
As the berries are shaken, gaps open up between and beneath them—some large and some small gaps. Since only small berries can move down into the small gaps, over time the large berries are nudged to the top.
Interestingly enough, even denser objects move to the top in this way. Gentle motions in the ground nudge heavy rocks to the surface—a source of frustration to gardeners.
1. sink to the bottom. 2. go to the top. 3. not particularly sink or rise, but like the smaller berries, be randomly distributed.

5. Which will roll down a hill faster, a can of regular fruit juice or a can of frozen fruit juice?
1. Regular fruit juice 2. Frozen fruit juice 3. Depends on the relative sizes and weights of the cans

6. Which will roll down a hill faster, a can of regular fruit juice or a can of frozen fruit juice?
Answer: 1
The regular fruit juice has an appreciably greater acceleration down an incline than the can of frozen juice. Why? Because the regular juice is a liquid and is not made to roll with the can, as the solid juice does. Most of the liquid effectively slides down the incline inside the rolling can. The can of liquid therefore has very little rotational inertia compared to its mass. The solid juice, on the other hand, is made to rotate, giving the can more rotational inertia.
1. Regular fruit juice 2. Frozen fruit juice 3. Depends on the relative sizes and weights of the cans

7. Roll a pair of identical cans of carbonated beverage down an incline
Roll a pair of identical cans of carbonated beverage down an incline. You won’t be surprised to find they roll at the same rate. Now shake one of them so bubbles form inside, then repeat the experiment. You’ll be delighted to observe that
Ch 8-5
Thanks to Robin McGlohn and David Kagan.
1. the shaken can wins the race. 2. the shaken can loses the race. 3. both cans still roll together.

8. Roll a pair of identical cans of carbonated beverage down an incline
Roll a pair of identical cans of carbonated beverage down an incline. You won’t be surprised to find they roll at the same rate. Now shake one of them so bubbles form inside, then repeat the experiment. You’ll be delighted to observe that
Ch 8-5
Thanks to Robin McGlohn and David Kagan.
Answer: 2
The shaken can rolls slower and loses the race. Why? Suppose friction were practically absent between the contained liquid and the inner can surface. The metal can would then roll down the incline while the liquid inside would simply slide down without rolling. The liquid’s kinetic energy would all be translational (which is why a can of liquid always beats a can filled with solid material on the same incline). So what slows the can that is shaken? Perhaps surface tension between bubbles and the can creates more friction than that of straight fluid-can adhesion. Then the liquid would undergo some rotation, having rotational kinetic energy that diminishes translational kinetic energy.
1. the shaken can wins the race. 2. the shaken can loses the race. 3. both cans still roll together.

9. 1. weighted stick. 2. bare stick. 3. … both the same.
A pair of upright metersticks, with their lower ends against a wall, are allowed to fall to the floor. One is bare, and the other has a heavy weight attached to its upper end. The stick to hit the floor first is the
Ch 8-7
1. weighted stick. 2. bare stick. 3. … both the same.

10. 1. weighted stick. 2. bare stick. 3. … both the same.
A pair of upright metersticks, with their lower ends against a wall, are allowed to fall to the floor. One is bare, and the other has a heavy weight attached to its upper end. The stick to hit the floor first is the
Ch 8-7
Answer: 2
In falling, both metersticks rotate about an axis at the lower end where the wall and floor meet. Their rate of rotation depends on their rotational inertias. The meterstick with the heavy weight at its upper end has more rotational inertia and is more lazy in rotating about its lower end. So the bare meterstick rotates to the floor in the shortest time.
1. weighted stick. 2. bare stick. 3. … both the same.

11. 1. Short broom part 2. Long broom part
The broom balances at its center of gravity. If you saw the broom into two parts through the center of gravity and then weigh each part on a scale, which part will weigh more?
Ch 8-9
Thanks to Iain MacInnes.
1. Short broom part 2. Long broom part

12. 1. Short broom part 2. Long broom part
The broom balances at its center of gravity. If you saw the broom into two parts through the center of gravity and then weigh each part on a scale, which part will weigh more?
Ch 8-9
Thanks to Iain MacInnes.
Answer: 1
The short broom part is heavier. It balances the long handle just as kids of unequal weights can balance on a seesaw when the heavier kid sits closer to the fulcrum. Both the balanced broom and seesaw are evidence of equal and opposite torques—not equal weights.
1. Short broom part 2. Long broom part

13. When a tether ball wraps around a pole, the speed of the ball
Ch 8-13
1. increases. 2. decreases. 3. remains unchanged.

14. 1. increases. 2. decreases. 3. remains unchanged.
When a tether ball wraps around a pole, the speed of the ball
Ch 8-13
Answer: 3
The speed doesn’t change because the instantaneous velocity of the ball is perpendicular to the cord at all times. The tension in the cord, then, does no work on the ball and cannot change its kinetic energy. So the speed remains constant. If you answered 1 you likely thought of angular momentum conservation. But angular momentum is conserved only in the absence of a torque. Relative to the center of the pole, the cord exerts a torque on the ball, causing it to lose angular momentum. As the ball spirals in, its radius and angular momentum decrease while its speed doesn’t change.
1. increases. 2. decreases. 3. remains unchanged.

15. 1. Same as before. 2. Opposite. 3. Not at all.
Inverse Lawn Sprinkler With a nail poke four holes in an aluminum pop can as shown. In each hole bend the nail sideways and dent the holes so that when water is put in the can it will spurt out with a tangential component. Suspend the can with strings and watch it rotate as water spurts from it—noting its direction of rotation. Now empty the can; weigh it down at its bottom so that when you suspend it in water it remains upright as water flows into the holes. Question: What's the direction of rotation?
Ch 8-14
Thanks to Robert Ehrlich and Peter Hopkinson.
1. Same as before. 2. Opposite. 3. Not at all.

16. 1. Same as before. 2. Opposite. 3. Not at all.
Inverse Lawn Sprinkler With a nail poke four holes in an aluminum pop can as shown. In each hole bend the nail sideways and dent the holes so that when water is put in the can it will spurt out with a tangential component. Suspend the can with strings and watch it rotate as water spurts from it—noting its direction of rotation. Now empty the can; weigh it down at its bottom so that when you suspend it in water it remains upright as water flows into the holes. Question: What's the direction of rotation?
Ch 8-14
Thanks to Robert Ehrlich and Peter Hopkinson.
Answer: 2
In Case 1, water is pushed radially outward due to pressure difference from inside to outside the can. But on its way out it is deflected by the dented hole and gains a tangential component of velocity. In action–reaction fashion, the water pushes tangentially back on the can, giving it a torque that sets it into rotation. In Case 2, the water is also deflected by the dented hole as it enters the can, but in the opposite direction. This time the water’s reaction force on the can gives the can a torque opposite to that of Case 1, rotating the can in the opposite direction. (In Case 1, the can, once set rotating, slows down gradually because very little friction acts on it. In Case 2, friction of the swirling water with the inside surface of the can applies a torque that slows the rotation more quickly.)
1. Same as before. 2. Opposite. 3. Not at all.