Presentation on theme: "2. The two carts are identical. 3. Cart B is hollow."— Presentation transcript:
1 2. The two carts are identical. 3. Cart B is hollow. You are given two carts, A and B. They look identical, and you are told that they are made of the same material. You place A at rest on an air track and give B a constant velocity directed to the right so that it collides with A. After the collision, both carts move to the right, the velocity of B being smaller than what it was before the collision. Whatdo you conclude?1. Cart A is hollow.2. The two carts are identical.3. Cart B is hollow.4. need more information1, otherwise B would either bounce back (A heavier) or stay in the same place (A equal to B)
2 A car accelerates from rest. In doing so the car gains a certain amount of momentumand Earth gains1. more momentum.2. the same amount of momentum.3. less momentum.4. The answer depends on the interactionbetween the two.2. Momentum is conserved (always!)
3 1. gains more kinetic energy. A car accelerates from rest. It gains a certain amount of kinetic energy and Earth1. gains more kinetic energy.2. gains the same amount of kinetic energy.3. gains less kinetic energy.4. loses kinetic energy as the car gains it.3. Given that the momentum change, mv, is equal and opposite, the kinetic energy change, mv2, can’t also be equal and opposite, unless the masses are the same, which they are not. If you write out the equations and plug in reasonable numbers (or just think about it) you’ll see that it’s answer 3 and not answer 1. Note, total energy is still conserved (always!), but keeping track of energy means finding out what’s powering the car, etc. not just worrying about the change in kinetic energy of the two objects.
4 Suppose the entire population of the world gathers in one spot and, at the sounding of a prearranged signal, everyone jumps up. While all the people are in the air, does Earth gain momentum in the opposite direction?1. No; the inertial mass of Earth is so large that the planet’s change in motion is zero.2. Yes; because of its much larger inertial mass, however, the change in momentum of Earth is much less than that of all the jumping people.3. Yes; Earth recoils, like a rifle firing a bullet, with a change in momentum equal to and opposite that of the people.4. It depends.3. Momentum is conserved (always)
5 second later,5 billion people land back on the Suppose the entire population of the world gathers in one spot and, at the sound of a prearranged signal, everyone jumps up. About asecond later,5 billion people land back on theground. After the people have landed, Earth’smomentum is1. the same as what it was before the peoplejumped.2. different from what it was before the people jumped.1. You can’t change the momentum of a system (e.g. earth+people) from 100% within that system. You’d need to throw something outside of the system (e.g. expanding rocket fuel exhaust) in order to move the system.
6 Suppose rain falls vertically into an open cart rolling along a straight horizontal trackwith negligible friction. As a result of theaccumulating water, the momentum of the cart1. increases.2. does not change.3. decreases.2. Collisions with vertically falling rain can’t change horizontal momentum
7 Suppose rain falls vertically into an open cart rolling along a straight horizontal trackwith negligible friction. As a result of theaccumulating water, the speed of the cart1. increases.2. does not change.3. decreases.3. More mass but equal horizontal momentum means less speed
8 Suppose rain falls vertically into an open cart rolling along a straight horizontal track with negligible friction. As a result of the accumulating water, the kinetic energy of the cart1. increases.2. does not change.3. decreases.3. Since the speed decreases by the same factor that the mass increases and since the kinetic energy goes as the square of the speed but only linearly with mass, the kinetic energy decreases (energy is conserved because the inelastic collision of the rain heats things up)
9 Consider these situations: (i) a ball moving at speed v is brought to rest;(ii) the same ball is projected from rest so that it moves at speed v;(iii) the same ball moving at speed v is brought to rest and then projected backward to its original speed.In which case(s) does the ball undergo the largest change in momentum?1. (i)2. (i) and (ii)3. (ii)4. (ii) and (iii)5. (iii)5. Because you get 2 changes in mv for the price of 1 . Hitting the wall is like being brought to rest from speed v, bouncing back is like being projected from rest back to speed v.
10 Consider two carts, of masses m and 2m, at rest on an air track. If you push first one cartfor 3 s and then the other for the same lengthof time, exerting equal force on each, themomentum of the light cart is1. four times2. twice3. equal to4. one-half5. one-quarterthe momentum of the heavy cart.3
11 Consider two carts, of masses m and 2m, at rest on an air track. If you push first one cartfor 3 s and then the other for the same lengthof time, exerting equal force on each, the kineticenergy of the light cart is1. larger than2. equal to3. smaller thanthe kinetic energy of the heavy car.1
12 Suppose a ping-pong ball and a bowling ball are rolling toward you. Both have the samemomentum, and you exert the same force tostop each. How do the time intervals to stopthem compare?1. It takes less time to stop the ping-pong ball.2. Both take the same time.3. It takes more time to stop the ping-pongball.2
13 Suppose a ping-pong ball and a bowling ball are rolling toward you. Both have the samemomentum, and you exert the same force tostop each. How do the distances needed tostop them compare?1. It takes a shorter distance to stop the ping-pong ball.2. Both take the same distance.3. It takes a longer distance to stop the ping-pong ball.3
14 If ball 1 in the arrangement shown here is pulled back and then let go, ball 5 bouncesforward. If balls 1 and 2 are pulled back andreleased, balls 4 and 5 bounce forward, andso on. The number of balls bouncing on eachside is equal because1. of conservation of momentum.2. the collisions are all elastic.3. neither of the above2
15 A cart moving at speed v collides with an identical stationary cart on an airtrack, andthe two stick together after the collision. Whatis their velocity after colliding?1. vv3. zero4. –0.5 v5. –v6. need more information2
16 A person attempts to knock down a large wooden bowling pin by throwing a ball at it.The person has two balls of equal size andmass, one made of rubber and the other ofputty. The rubber ball bounces back, while theball of putty sticks to the pin. Which ball ismost likely to topple the bowling pin?1. the rubber ball2. the ball of putty3. makes no difference4. need more information1
17 Think fast! You’ve just driven around a curve in a narrow, one-way street at 25 mph whenyou notice a car identical to yours comingstraight toward you at 25 mph. You have onlytwo options: hitting the other car head on orswerving into a massive concrete wall, alsohead on. In the split second before the impact,you decide to1. hit the other car.2. hit the wall.3. hit either one—it makes no difference.4. consult your lecture notes.3
18 If all three collisions in the figure shown here are totally inelastic, which bring(s) thecar on the left to a halt?1. I2. II3. III4. I, II5. I, III6. II, III7. all three7.
19 If all three collisions in the figure shown are totally inelastic, which cause(s) themost damage?1. I2. II3. III4. I, II5. I, III6. II, III7. all three3
20 A golf ball is fired at a bowling ball initially at rest and bounces back elastically. Comparedto the bowling ball, the golf ball afterthe collision has1. more momentum but less kinetic energy.2. more momentum and more kinetic energy.3. less momentum and less kinetic energy.4. less momentum but more kinetic energy.5. none of the above4
21 A golf ball is fired at a bowling ball initially at rest and sticks to it. Compared to the bowlingball, the golf ball after the collision has1. more momentum but less kinetic energy.2. more momentum and more kinetic energy.3. less momentum and less kinetic energy.4. less momentum but more kinetic energy.5. none of the above3
22 Suppose you are on a cart, initially at rest on a track with very little friction. You throwballs at a partition that is rigidly mounted onthe cart. If the balls bounce straight back asshown in the figure, is the cart put in motion?1. Yes, it moves to the right.2. Yes, it moves to the left.3. No, it remains in place.2
23 A compact car and a large truck collide head on and stick together. Which undergoes thelarger momentum change?1. car2. truck3. The momentum change is the same for both vehicles.4. Can’t tell without knowing the final velocityof combined mass.3
24 A compact car and a large truck collide head on and stick together. Which vehicleundergoes the larger acceleration duringthe collision?1. car2. truck3. Both experience the same acceleration.4. Can’t tell without knowing the final velocity of combined mass.1
25 Is it possible for a stationary object that is struck by a moving object to have a largerfinal momentum than the initial momentumof the incoming object?1. Yes.2. No because such an occurrence wouldviolate the law of conservation of momentum.1
26 1. A is put in motion but B remains at rest. Two carts of identical inertial mass are put back-to-back on a track. Cart A has a spring loaded piston; cart B is entirely passive. When the piston is released, it pushes against cart B, and1. A is put in motion but B remains at rest.2. both carts are set into motion, with A gaining more speed than B.3. both carts gain equal speed but in opposite directions.4. both carts are set into motion, with B gaining more speed than A.5. B is put in motion but A remains at rest.3
27 Two carts are put back-to-back on a track. Cart A has a spring-loaded piston; cart B,which has twice the inertial mass of cart A, isentirely passive. When the piston is released,it pushes against cart B, and the carts moveapart. How do the magnitudes of the finalmomenta and kinetic energies compare?1. pA > pB, kA > kB2. pA > pB, kA = kB3. pA > pB, kA < kB4. pA = pB, kA > kB5. pA = pB, kA = kB6. pA = pB, kA < kB7. pA < pB, kA > kB8. pA < pB, kA = kB9. pA < pB, kA < kB4
28 Two carts are put back-to-back on a track. Cart A has a spring-loaded piston; cart B,which has twice the inertial mass of cart A,is entirely passive. When the piston is released,it pushes against cart B, and the cartsmove apart. Ignoring signs, while the pistonis pushing,1. A has a larger acceleration than B.2. the two have the same acceleration.3. B has a larger acceleration than A.1
29 Two people on roller blades throw a ball back and forth. Which statement(s) is/are true?A. The interaction mediated by the ball isrepulsive.B. If we film the action and play the moviebackward, the interaction appears attractive.C. The total momentum of the two peopleis conserved.D. The total energy of the two people isconserved.What about the conservation laws? The ball carries both momentum and energy back and forth between the two roller-bladers. Their momentum and energy therefore cannot be conserved.4
30 1. reverses its direction of travel well above the eggs. In the following figure, a 10-kg weight is suspended from the ceiling by a spring. The weight-spring system is at equilibrium with the bottom of the weight about 1 m above the floor. The spring is then stretched until the weight is just above the eggs. When the spring is released, the weight is pulled up by the contracting spring and then falls back down under the influence of gravity. On the way down, it1. reverses its direction of travel well above the eggs.2. reverses its direction of travel precisely as it reaches the eggs.3. makes a mess as it crashes into the eggs.2
31 In part (a) of the figure, an air track cart attached to a spring rests on the track at the position xequilibrium and the spring is relaxed. In (b), the cart is pulled to the position xstart and released. It then oscillates aboutxequilibrium.Which graph correctly represents the potential energy of the spring as a function of theposition of the cart?3