1. Forces

2. Three forces act on an object
Three forces act on an object. In which direction does the object accelerate?
STT4.4

3. Three forces act on an object
Three forces act on an object. In which direction does the object accelerate?
STT4.4

4. An elevator suspended by a cable is moving upward and slowing to a stop. Which free-body diagram is correct?
STT4.5

5. An elevator suspended by a cable is moving upward and slowing to a stop. Which free-body diagram is correct?
The acceleration must be down, so the net force must be down.
STT4.5

6. A Martian lander is approaching the surface
A Martian lander is approaching the surface. It is slowing its descent by firing its rocket motor. Which is the correct free-body diagram for the lander?
STT5.1

7. A Martian lander is approaching the surface
A Martian lander is approaching the surface. It is slowing its descent by firing its rocket motor. Which is the correct free-body diagram for the lander?
To slow down while descending, the acceleration must be up. To accelerate up, the net force must be up. Choice a is the only free body diagram consistent with that determination.
STT5.1

8. Gravity, acting downward. The normal force, acting upward.
You’ve just kicked a rock, and it is now sliding across the ground about 2 meters in front of you. Which of these forces act on the ball? Check all that apply.
Gravity, acting downward.
The normal force, acting upward.
The force of the kick, acting in the direction of motion.
Friction, acting opposite the direction of motion.
STT4.2

9. Gravity, acting downward. The normal force, acting upward.
You’ve just kicked a rock, and it is now sliding across the ground about 2 meters in front of you. Which of these forces act on the ball?
Gravity, acting downward.
The normal force, acting upward.
The force of the kick, acting in the direction of motion.
Friction, acting opposite the direction of motion.
STT4.2
Once the foot is no longer in contact with the rock, it no longer exerts a force on the rock

10. Rank order, from largest to smallest, the size of the
friction forces to
in these 5 different situations.
The box and the floor are made of the same materials in all situations.
fc > fd > fe > fb > fa.
fb > fc > fd > fe > fa.
fa > fc = fd = fe > fb.
fa = fb > fc = fd = fe.
fb > fc = fd = fe > fa.
STT5.3

11. Rank order, from largest to smallest, the size of the
friction forces to
in these 5 different situations.
The box and the floor are made of the same materials in all situations.
fc > fd > fe > fb > fa.
fb > fc > fd > fe > fa.
fa > fc = fd = fe > fb.
fa = fb > fc = fd = fe.
fb > fc = fd = fe > fa.
STT5.3
The is no friction in case a. Case b is near maximum static friction which is larger than kinetic friction. Kinetic friction does not depend on speed.

12. A car is rolling over the top of a hill at speed v. At this instant,
n > w.
n = w.
n < w.
We can’t tell about n without knowing v.
STT7.4

13. A car is rolling over the top of a hill at speed v. At this instant,
n > w.
n = w.
n < w.
We can’t tell about n without knowing v.
STT7.4
The acceleration is towards the center of the circle, in this case down. The net force must be down by Newton’s second law.

14. A ball on a string is swung in a vertical circle
A ball on a string is swung in a vertical circle. The string happens to break when it is parallel to the ground and the ball is moving up. Which trajectory does the ball follow?
STT7.5

15. A ball on a string is swung in a vertical circle
A ball on a string is swung in a vertical circle. The string happens to break when it is parallel to the ground and the ball is moving up. Which trajectory does the ball follow?
The velocity when the string breaks is straight up. The only force acting on the ball is gravity which has no x component, so there is no subsequent motion in the x direction.
STT7.5

16. B exerts a force on A but A doesn’t exert a force on B.
Car B is stopped for a red light. Car A, which has the same mass as car B, doesn’t see the red light and runs into the back of B. Which of the following statements is true?
B exerts a force on A but A doesn’t exert a force on B.
B exerts a larger force on A than A exerts on B.
B exerts the same amount of force on A as A exerts on B.
A exerts a larger force on B than B exerts on A.
A exerts a force on B but B doesn’t exert a force on A.
STT8.2

17. B exerts a force on A but A doesn’t exert a force on B.
Car B is stopped for a red light. Car A, which has the same mass as car B, doesn’t see the red light and runs into the back of B. Which of the following statements is true?
B exerts a force on A but A doesn’t exert a force on B.
B exerts a larger force on A than A exerts on B.
B exerts the same amount of force on A as A exerts on B.
A exerts a larger force on B than B exerts on A.
A exerts a force on B but B doesn’t exert a force on A.
STT8.2
This is true by Newton’s third law.

18. FB on H = FH on B = FA on B = FB on A
Boxes A and B are sliding to the right across a frictionless table. The hand H is slowing them down. The mass of A is larger than the mass of B. Rank in order, from largest to smallest, the horizontal forces on A, B, and H.
STT8.3
FB on H = FH on B = FA on B = FB on A
FB on H = FH on B > FA on B = FB on A
FB on H = FH on B < FA on B = FB on A
FH on B = FH on A > FA on B

19. FB on H = FH on B = FA on B = FB on A
Boxes A and B are sliding to the right across a frictionless table. The hand H is slowing them down. The mass of A is larger than the mass of B. Rank in order, from largest to smallest, the horizontal forces on A, B, and H.
STT8.3
FB on H = FH on B = FA on B = FB on A
FB on H = FH on B > FA on B = FB on A
FB on H = FH on B < FA on B = FB on A
FH on B = FH on A > FA on B
The equalities are true by Newton’s third law. Block B must be accelerating to the left, so the net force is to the left. This makes FHB > FAB.

20. All three 50 kg blocks are at rest
All three 50 kg blocks are at rest. Is the tension in rope 2 greater than, less than or equal to the tension in rope 1?
greater than
less than
equal to
STT8.4

21. All three 50 kg blocks are at rest
All three 50 kg blocks are at rest. Is the tension in rope 2 greater than, less than or equal to the tension in rope 1?
greater than
less than
equal to
Newton’s first law for the block on the left proves that the tension equals the weight. Newton’s first law for either of the individual blocks on the right proves that the tension equals the weight for an individual.
STT8.4

22. The block on the far right is moving up with constant speed
The block on the far right is moving up with constant speed. Is the tension in rope 2 greater than, less than or equal to the tension in rope 1?
greater than
less than
equal to
STT8.4

23. The block on the far right is moving up with constant speed
The block on the far right is moving up with constant speed. Is the tension in rope 2 greater than, less than or equal to the tension in rope 1?
greater than
less than
equal to
STT8.4
This is still a Newton’s first law situation!

24. In the figure to the right is the tension in the string greater than,
less than, or equal to the weight of block B?
Greater than
Less than
Equal to
STT8.5

25. In the figure to the right is the tension in the string greater than,
less than, or equal to the weight of block B?
Greater than
Less than
Equal to
STT8.5
This is a Newton’s second law situation for each of the blocks. Block A will accelerate to the right and block B will accelerate down. The net force on B must be down by Newton’s second law. The tension force exerted by the rope on block B must be less than the weight force exerted by the earth on block B.

26. the mass of the car. Which of the following statements is true?
A small car is pushing a larger truck that has a dead battery. The mass of the truck is larger than
the mass of the car. Which of the following statements is true?
The car exerts a force on the truck but the truck doesn’t exert a force on the car.
The car exerts a larger force on the truck than the truck exerts on the car.
The car exerts the same amount of force on the truck as the truck exerts on the car.
The truck exerts a larger force on the car than the car exerts on the truck.
The truck exerts a force on the car but the car doesn’t exert a force on the truck.
STT8.6

27. the mass of the car. Which of the following statements is true?
A small car is pushing a larger truck that has a dead battery. The mass of the truck is larger than
the mass of the car. Which of the following statements is true?
The car exerts a force on the truck but the truck doesn’t exert a force on the car.
The car exerts a larger force on the truck than the truck exerts on the car.
The car exerts the same amount of force on the truck as the truck exerts on the car.
The truck exerts a larger force on the car than the car exerts on the truck.
The truck exerts a force on the car but the car doesn’t exert a force on the truck.
STT8.6
This is true by Newton’s third law.