1. ConcepTest Clicker Questions College Physics, 7th Edition
Chapter 3
College Physics, 7th Edition
Wilson / Buffa / Lou
© 2010 Pearson Education, Inc.

2. ConcepTest 6.1 Rolling in the Rain
An open cart rolls along a frictionless track while it is raining. As it rolls, what happens to the speed of the cart as the rain collects in it? (Assume that the rain falls vertically into the box.)
a) speeds up
b) maintains constant speed
c) slows down
d) stops immediately
Answer: c

3. ConcepTest 6.1 Rolling in the Rain
An open cart rolls along a frictionless track while it is raining. As it rolls, what happens to the speed of the cart as the rain collects in it? (Assume that the rain falls vertically into the box.)
a) speeds up
b) maintains constant speed
c) slows down
d) stops immediately
Because the rain falls in vertically, it adds no momentum to the box, thus the box’s momentum is conserved. However, because the mass of the box slowly increases with the added rain, its velocity has to decrease.
Follow-up: What happens to the cart when it stops raining?

4. Question 6.4 Collision Course
a) the car
b) the truck
c) they both have the same momentum change
d) can’t tell without knowing the final velocities
A small car and a large truck collide head-on and stick together. Which one has the larger momentum change?
Answer: c

5. Question 6.4 Collision Course
a) the car
b) the truck
c) they both have the same momentum change
d) can’t tell without knowing the final velocities
A small car and a large truck collide head-on and stick together. Which one has the larger momentum change?
Because the total momentum of the because is conserved, that means that Dp = 0 for the car and truck combined. Therefore, Dpcar must be equal and opposite to that of the truck (–Dptruck) in order for the total momentum change to be zero. Note that this conclusion also follows from Newton’s Third Law.
Follow-up: Which one feels the larger acceleration?

6. Question 6.5a Two Boxes I
Two boxes, one heavier than the other, are initially at rest on a horizontal frictionless surface. The same constant force F acts on each one for exactly 1 second. Which box has more momentum after the force acts ?
a) the heavier one
b) the lighter one
c) both the same
Answer: c F
light
heavy

7. Question 6.5a Two Boxes I Dp = F Dt , F
Two boxes, one heavier than the other, are initially at rest on a horizontal frictionless surface. The same constant force F acts on each one for exactly 1 second. Which box has more momentum after the force acts ?
a) the heavier one
b) the lighter one
c) both the same av Dt Dp F = ,
We know: F
light
heavy
so impulse Dp = Fav Dt.
In this case F and Dt are the same for both boxes!
Both boxes will have the same final momentum.

8. Question 6.5b Two Boxes II
In the previous question, which box has the larger velocity after the force acts?
a) the heavier one
b) the lighter one
c) both the same
Answer: b

9. Question 6.5b Two Boxes II
In the previous question, which box has the larger velocity after the force acts?
a) the heavier one
b) the lighter one
c) both the same
The force is related to the acceleration by Newton’s Second Law (F = ma). The lighter box therefore has the greater acceleration and will reach a higher speed after the 1-second time interval.
Follow-up: Which box has gone a larger distance after the force acts?
Follow-up: Which box has gained more KE after the force acts?

10. Question Impulse
A small beanbag and a bouncy rubber ball are dropped from the same height above the floor. They both have the same mass. Which one will impart the greater impulse to the floor when it hits?
a) the beanbag
b) the rubber ball
c) both the same
Answer: b

11. Question Impulse
A small beanbag and a bouncy rubber ball are dropped from the same height above the floor. They both have the same mass. Which one will impart the greater impulse to the floor when it hits?
a) the beanbag
b) the rubber ball
c) both the same
Both objects reach the same speed at the floor. However, while the beanbag comes to rest on the floor, the ball bounces back up with nearly the same speed as it hit. Thus, the change in momentum for the ball is greater, because of the rebound. The impulse delivered by the ball is twice that of the beanbag.
For the beanbag: Dp = pf – pi = 0 – (–mv ) = mv
For the rubber ball: Dp = pf – pi = mv – (–mv ) = 2mv
Follow-up: Which one imparts the larger force to the floor?

12. Question 6.9a Going Bowling I
A bowling ball and a Ping-Pong ball are rolling toward you with the same momentum. If you exert the same force to stop each one, which takes a longer time to bring to rest?
a) the bowling ball
b) same time for both
c) the Ping-Pong ball
d) impossible to say p
Answer: b

13. Question 6.9a Going Bowling I
A bowling ball and a Ping-Pong ball are rolling toward you with the same momentum. If you exert the same force to stop each one, which takes a longer time to bring to rest?
a) the bowling ball
b) same time for both
c) the Ping-Pong ball
d) impossible to say av Dt Dp F =
We know: p
so Dp = Fav Dt
Here, F and Dp are the same for both balls!
It will take the same amount of time to stop them.

14. Question 6.14a Recoil Speed I
Amy (150 lbs) and Gwen (50 lbs) are standing on slippery ice and push off each other. If Amy slides at 6 m/s, what speed does Gwen have?
a) 2 m/s
b) 6 m/s
c) 9 m/s
d) 12 m/s
e) 18 m/s
150 lbs
50 lbs
Answer: e

15. Question 6.14a Recoil Speed I
Amy (150 lbs) and Gwen (50 lbs) are standing on slippery ice and push off each other. If Amy slides at 6 m/s, what speed does Gwen have?
a) 2 m/s
b) 6 m/s
c) 9 m/s
d) 12 m/s
e) 18 m/s
The initial momentum is zero, so the momenta of Amy and Gwen must be equal and opposite. Because p = mv, then if Amy has three times more mass, we see that Gwen must have three times more speed.
150 lbs
50 lbs

16. Question 6.14b Recoil Speed II
A cannon sits on a stationary railroad flatcar with a total mass of 1000 kg. When a 10-kg cannonball is fired to the left at a speed of 50 m/s, what is the recoil speed of the flatcar?
a) 0 m/s
b) 0.5 m/s to the right
c) 1 m/s to the right
d) 20 m/s to the right
e) 50 m/s to the right
Answer: b

17. Question 6.14b Recoil Speed II
A cannon sits on a stationary railroad flatcar with a total mass of 1000 kg. When a 10-kg cannonball is fired to the left at a speed of 50 m/s, what is the recoil speed of the flatcar?
a) 0 m/s
b) 0.5 m/s to the right
c) 1 m/s to the right
d) 20 m/s to the right
e) 50 m/s to the right
Because the initial momentum of the system was zero, the final total momentum must also be zero. Thus, the final momenta of the cannonball and the flatcar must be equal and opposite.
pcannonball = (10 kg)(50 m/s) = 500 kg-m/s
pflatcar = 500 kg-m/s = (1000 kg)(0.5 m/s)

18. Question Shut the Door!
You are lying in bed and you want to shut your bedroom door. You have a superball and a blob of clay (both with the same mass) sitting next to you. Which one would be more effective to throw at your door to close it?
a) the superball
b) the blob of clay
c) it doesn’t matter—they will be equally effective
d) you are just too lazy to throw anything
Answer: a

19. Question Shut the Door!
You are lying in bed and you want to shut your bedroom door. You have a superball and a blob of clay (both with the same mass) sitting next to you. Which one would be more effective to throw at your door to close it?
a) the superball
b) the blob of clay
c) it doesn’t matter—they will be equally effective
d) you are just too lazy to throw anything
The superball bounces off the door with almost no loss of speed, so its Dp (and that of the door) is 2mv.
The clay sticks to the door and continues to move along with it, so its Dp is less than that of the superball, and therefore it imparts less Dp to the door.

20. Question 3.4a Firing Balls I
A small cart is rolling at constant velocity on a flat track. It fires a ball straight up into the air as it moves. After it is fired, what happens to the ball?
a) it depends on how fast the cart is moving
b) it falls behind the cart
c) it falls in front of the cart
d) it falls right back into the cart
e) it remains at rest
Answer: d

21. Question 3.4a Firing Balls I
A small cart is rolling at constant velocity on a flat track. It fires a ball straight up into the air as it moves. After it is fired, what happens to the ball?
a) it depends on how fast the cart is moving
b) it falls behind the cart
c) it falls in front of the cart
d) it falls right back into the cart
e) it remains at rest
In the frame of reference of the cart, the ball only has a vertical component of velocity. So it goes up and comes back down. To a ground observer, both the cart and the ball have the same horizontal velocity, so the ball still returns into the cart.
when viewed from train
when viewed from ground

22. Question 3.4b Firing Balls II
Now the cart is being pulled along a horizontal track by an external force (a weight hanging over the table edge) and accelerating. It fires a ball straight out of the cannon as it moves. After it is fired, what happens to the ball?
a) it depends upon how much the track is tilted
b) it falls behind the cart
c) it falls in front of the cart
d) it falls right back into the cart
e) it remains at rest
Answer: b

23. Question 3.4b Firing Balls II
Now the cart is being pulled along a horizontal track by an external force (a weight hanging over the table edge) and accelerating. It fires a ball straight out of the cannon as it moves. After it is fired, what happens to the ball?
a) it depends upon how much the track is tilted
b) it falls behind the cart
c) it falls in front of the cart
d) it falls right back into the cart
e) it remains at rest
Now the acceleration of the cart is completely unrelated to the ball. In fact, the ball does not have any horizontal acceleration at all (just like the first question), so it will lag behind the accelerating cart once it is shot out of the cannon.

24. Question 3.5 Dropping a Package
a) quickly lag behind the plane while falling
b) remain vertically under the plane while falling
c) move ahead of the plane while falling
d) not fall at all
You drop a package from a plane flying at constant speed in a straight line. Without air resistance, the package will:
Answer: b

25. Question 3.5 Dropping a Package
a) quickly lag behind the plane while falling
b) remain vertically under the plane while falling
c) move ahead of the plane while falling
d) not fall at all
You drop a package from a plane flying at constant speed in a straight line. Without air resistance, the package will:
Both the plane and the package have the same horizontal velocity at the moment of release. They will maintain this velocity in the x-direction, so they stay aligned.
Follow-up: what would happen if air resistance is present?

26. Question 3.6a Dropping the Ball I
a) the “dropped” ball
b) the “fired” ball
c) they both hit at the same time
d) it depends on how hard the ball was fired
e) it depends on the initial height
From the same height (and at the same time), one ball is dropped and another ball is fired horizontally. Which one will hit the ground first?
Answer: c

27. Question 3.6a Dropping the Ball I
a) the “dropped” ball
b) the “fired” ball
c) they both hit at the same time
d) it depends on how hard the ball was fired
e) it depends on the initial height
From the same height (and at the same time), one ball is dropped and another ball is fired horizontally. Which one will hit the ground first?
Both of the balls are falling vertically under the influence of gravity. They both fall from the same height. Therefore, they will hit the ground at the same time. The fact that one is moving horizontally is irrelevant—remember that the x and y motions are completely independent !!
Follow-up: is that also true if there is air resistance?

28. Question 3.6b Dropping the Ball II
a) the “dropped” ball
b) the “fired” ball
c) neither—they both have the same velocity on impact
d) it depends on how hard the ball was thrown
In the previous problem, which ball has the greater velocity at ground level?
Answer: b

29. Question 3.6b Dropping the Ball II
a) the “dropped” ball
b) the “fired” ball
c) neither—they both have the same velocity on impact
d) it depends on how hard the ball was thrown
In the previous problem, which ball has the greater velocity at ground level?
Both balls have the same vertical velocity when they hit the ground (since they are both acted on by gravity for the same time). However, the “fired” ball also has a horizontal velocity. When you add the two components vectorially, the “fired” ball has a larger net velocity when it hits the ground.
Follow-up: what would you have to do to have them both reach the same final velocity at ground level?

30. Question 3.6c Dropping the Ball III
a) just after it is launched
b) at the highest point in its flight
c) just before it hits the ground
d) halfway between the ground and the highest point
e) speed is always constant
A projectile is launched from the ground at an angle of 30°. At what point in its trajectory does this projectile have the least speed?
Answer: b

31. Question 3.6c Dropping the Ball III
a) just after it is launched
b) at the highest point in its flight
c) just before it hits the ground
d) halfway between the ground and the highest point
e) speed is always constant
A projectile is launched from the ground at an angle of 30º. At what point in its trajectory does this projectile have the least speed?
The speed is smallest at the highest point of its flight path because the y-component of the velocity is zero.

32. Question 3.7a Punts I
Which of the three punts has the longest hang time?
d) all have the same hang time a b c h
Answer: d

33. Question 3.7a Punts I
Which of the three punts has the longest hang time?
d) all have the same hang time a b c h
The time in the air is determined by the vertical motion! Because all of the punts reach the same height, they all stay in the air for the same time.
Follow-up: Which one had the greater initial velocity?

34. Question 3.7b Punts II a b c) both at the same time
A battleship simultaneously fires two shells at two enemy submarines. The shells are launched with the same initial velocity. If the shells follow the trajectories shown, which submarine gets hit first ? a b
Answer: b
c) both at the same time

35. Question 3.7b Punts II a b c) both at the same time
A battleship simultaneously fires two shells at two enemy submarines. The shells are launched with the same initial velocity. If the shells follow the trajectories shown, which submarine gets hit first ?
The flight time is fixed by the motion in the y-direction. The higher an object goes, the longer it stays in flight. The shell hitting submarine #2 goes less high, therefore it stays in flight for less time than the other shell. Thus, submarine #2 is hit first. a b
c) both at the same time
Follow-up: which one traveled the greater distance?

36. Question 3.8 Cannon on the Moon
For a cannon on Earth, the cannonball would follow path 2. Instead, if the same cannon were on the Moon, where g = 1.6 m/s2, which path would the cannonball take in the same situation? a b d c
Answer: d

37. Question 3.8 Cannon on the Moon
For a cannon on Earth, the cannonball would follow path 2. Instead, if the same cannon were on the Moon, where g = 1.6 m/s2, which path would the cannonball take in the same situation?
The ball will spend more time in flight because gMoon < gEarth. With more time, it can travel farther in the horizontal direction. a b d c
Follow-up: which path would it take in outer space?