1. ConcepTest 4.9a Going Up I v m 1) N > mg 2) N = mg
A block of mass m rests on the floor of an elevator that is moving upward at constant speed. What is the relationship between the force due to gravity and the normal force on the block?
1) N > mg
2) N = mg
3) N < mg (but not zero)
4) N = 0
5) depends on the size of the elevator m v

2. ConcepTest 4.9a Going Up I v m 1) N > mg 2) N = mg
A block of mass m rests on the floor of an elevator that is moving upward at constant speed. What is the relationship between the force due to gravity and the normal force on the block?
1) N > mg
2) N = mg
3) N < mg (but not zero)
4) N = 0
5) depends on the size of the elevator m v
The block is moving at constant speed, so it must have no net force on it. The forces on it are N (up) and mg (down), so N = mg, just like the block at rest on a table.

3. ConcepTest 4.9b Going Up II
A block of mass m rests on the floor of an elevator that is accelerating upward. What is the relationship between the force due to gravity and the normal force on the block?
1) N > mg
2) N = mg
3) N < mg (but not zero)
4) N = 0
5) depends on the size of the elevator m a
[CORRECT 5 ANSWER]

4. ConcepTest 4.9b Going Up II
A block of mass m rests on the floor of an elevator that is accelerating upward. What is the relationship between the force due to gravity and the normal force on the block?
1) N > mg
2) N = mg
3) N < mg (but not zero)
4) N = 0
5) depends on the size of the elevator
The block is accelerating upward, so it must have a net upward force. The forces on it are N (up) and mg (down), so N must be greater than mg in order to give the net upward force! m
a > 0 mg N
S F = N – mg = ma > 0
\ N > mg
Follow-up: What is the normal force if the elevator is in free fall downward?

5. ConcepTest 4.10 Normal Force
Below you see two cases: a physics student pulling or pushing a sled with a force F which is applied at an angle q. In which case is the normal force greater?
1) case 1
2) case 2
3) it’s the same for both
4) depends on the magnitude of the force F
5) depends on the ice surface
Case 1
Case 2

6. ConcepTest 4.10 Normal Force
Below you see two cases: a physics student pulling or pushing a sled with a force F which is applied at an angle q. In which case is the normal force greater?
1) case 1
2) case 2
3) it’s the same for both
4) depends on the magnitude of the force F
5) depends on the ice surface
Case 1
Case 2
In Case 1, the force F is pushing down (in addition to mg), so the normal force needs to be larger. In Case 2, the force F is pulling up, against gravity, so the normal force is lessened.

7. ConcepTest 4.11 On an Incline
Consider two identical blocks, one resting on a flat surface and the other resting on an incline. For which case is the normal force greater?
1) case A
2) case B
3) both the same (N = mg)
4) both the same (0 < N < mg)
5) both the same (N = 0) A B
[CORRECT 5 ANSWER]

8. ConcepTest 4.11 On an Incline
Consider two identical blocks, one resting on a flat surface and the other resting on an incline. For which case is the normal force greater?
1) case A
2) case B
3) both the same (N = mg)
4) both the same (0 < N < mg)
5) both the same (N = 0)
In Case A, we know that N = W. In Case B, due to the angle of the incline, N < W. In fact, we can see that N = W cos(q). y x N f A B q Wy W q

9. ConcepTest 4.12 Climbing the Rope
When you climb up a rope, the first thing you do is pull down on the rope. How do you manage to go up the rope by doing that??
1) this slows your initial velocity, which is already upward
2) you don’t go up, you’re too heavy
3) you’re not really pulling down – it just seems that way
4) the rope actually pulls you up
5) you are pulling the ceiling down

10. ConcepTest 4.12 Climbing the Rope
When you climb up a rope, the first thing you do is pull down on the rope. How do you manage to go up the rope by doing that??
1) this slows your initial velocity, which is already upward
2) you don’t go up, you’re too heavy
3) you’re not really pulling down – it just seems that way
4) the rope actually pulls you up
5) you are pulling the ceiling down
When you pull down on the rope, the rope pulls up on you!! It is actually this upward force by the rope that makes you move up! This is the “reaction” force (by the rope on you) to the force that you exerted on the rope. And voilá, this is Newton’s Third Law.

11. ConcepTest 4.13a Bowling vs. Ping-Pong I
1) the bowling ball exerts a greater force on the ping-pong ball
2) the ping-pong ball exerts a greater force on the bowling ball
3) the forces are equal
4) the forces are zero because they cancel out
5) there are actually no forces at all
In outer space, a bowling ball and a ping-pong ball attract each other due to gravitational forces. How do the magnitudes of these attractive forces compare?
F12
F21

12. ConcepTest 4.13a Bowling vs. Ping-Pong I
1) the bowling ball exerts a greater force on the ping-pong ball
2) the ping-pong ball exerts a greater force on the bowling ball
3) the forces are equal
4) the forces are zero because they cancel out
5) there are actually no forces at all
In outer space, a bowling ball and a ping-pong ball attract each other due to gravitational forces. How do the magnitudes of these attractive forces compare?
The forces are equal and opposite by Newton’s Third Law!
F12
F21

13. ConcepTest 4.13b Bowling vs. Ping-Pong II
1) they do not accelerate because they are weightless
2) accels. are equal, but not opposite
3) accelerations are opposite, but bigger for the bowling ball
4) accelerations are opposite, but bigger for the ping-pong ball
5) accelerations are equal and opposite
In outer space, gravitational forces exerted by a bowling ball and a ping-pong ball on each other are equal and opposite. How do their accelerations compare?
F12
F21

14. ConcepTest 4.13b Bowling vs. Ping-Pong II
1) they do not accelerate because they are weightless
2) accels. are equal, but not opposite
3) accelerations are opposite, but bigger for the bowling ball
4) accelerations are opposite, but bigger for the ping-pong ball
5) accelerations are equal and opposite
In outer space, gravitational forces exerted by a bowling ball and a ping-pong ball on each other are equal and opposite. How do their accelerations compare?
The forces are equal and opposite -- this is Newton’s Third Law!! But the acceleration is F/m and so the smaller mass has the bigger acceleration.
F12
F21
Follow-up: Where will the balls meet if they are released from this position?

15. ConcepTest 4.14a Collision Course I
1) the car
2) the truck
3) both the same
4) it depends on the velocity of each
5) it depends on the mass of each
A small car collides with a large truck. Which experiences the greater impact force?

16. ConcepTest 4.14a Collision Course I
1) the car
2) the truck
3) both the same
4) it depends on the velocity of each
5) it depends on the mass of each
A small car collides with a large truck. Which experiences the greater impact force?
According to Newton’s Third Law, both vehicles experience the same magnitude of force.

17. ConcepTest 4.14b Collision Course II
1) the car
2) the truck
3) both the same
4) it depends on the velocity of each
5) it depends on the mass of each
In the collision between the car and the truck, which has the greater acceleration?

18. ConcepTest 4.14b Collision Course II
1) the car
2) the truck
3) both the same
4) it depends on the velocity of each
5) it depends on the mass of each
In the collision between the car and the truck, which has the greater acceleration?
We have seen that both vehicles experience the same magnitude of force. But the acceleration is given by F/m so the car has the larger acceleration, since it has the smaller mass.

19. ConcepTest 4.15a Contact Force I
If you push with force F on either the heavy box (m1) or the light box (m2), in which of the two cases below is the contact force between the two boxes larger?
1) case A
2) case B
3) same in both cases F m2 m1 A F m2 m1 B

20. ConcepTest 4.15a Contact Force I
If you push with force F on either the heavy box (m1) or the light box (m2), in which of the two cases below is the contact force between the two boxes larger?
1) case A
2) case B
3) same in both cases F m2 m1 A
The acceleration of both masses together is the same in either case. But the contact force is the only force that accelerates m1 in case A (or m2 in case B). Since m1 is the larger mass, it requires the larger contact force to achieve the same acceleration. F m2 m1 B
Follow-up: What is the accel. of each mass?

21. ConcepTest 4.15b Contact Force II
Two blocks of masses 2m and m are in contact on a horizontal frictionless surface. If a force F is applied to mass 2m, what is the force on mass m ?
1) 2 F
2) F
3) 1/2 F
4) 1/3 F
5) 1/4 F 2m m F

22. ConcepTest 4.15b Contact Force II
Two blocks of masses 2m and m are in contact on a horizontal frictionless surface. If a force F is applied to mass 2m, what is the force on mass m ?
1) 2 F
2) F
3) 1/2 F
4) 1/3 F
5) 1/4 F
The force F leads to a specific acceleration of the entire system. In order for mass m to accelerate at the same rate, the force on it must be smaller! How small?? Let’s see... 2m m F
Follow-up: What is the acceleration of each mass?

23. ConcepTest 4.16a Tension I
You tie a rope to a tree and you pull on the rope with a force of 100 N. What is the tension in the rope?
1) 0 N
2) 50 N
3) N
4) N
5) N

24. ConcepTest 4.16a Tension I
You tie a rope to a tree and you pull on the rope with a force of 100 N. What is the tension in the rope?
1) 0 N
2) 50 N
3) N
4) N
5) N
The tension in the rope is the force that the rope “feels” across any section of it (or that you would feel if you replaced a piece of the rope). Since you are pulling with a force of 100 N, that is the tension in the rope.

25. ConcepTest 4.16b Tension II
Two tug-of-war opponents each pull with a force of 100 N on opposite ends of a rope. What is the tension in the rope?
1) 0 N
2) 50 N
3) N
4) N
5) N

26. ConcepTest 4.16b Tension II
Two tug-of-war opponents each pull with a force of 100 N on opposite ends of a rope. What is the tension in the rope?
1) 0 N
2) 50 N
3) N
4) N
5) N
This is literally the identical situation to the previous question. The tension is not 200 N !! Whether the other end of the rope is pulled by a person, or pulled by a tree, the tension in the rope is still 100 N !!