Force diagram of a still car

1- Shown below are force diagrams for three carts that have different masses and are being pushed forward by different strength fans. If these three carts were in a race, which one would speed up at the slowest rate and hence lose the race? Select the cart that you think will finish last in a race between them.

Select one:

a. Bottom cart

b. Middle cart

c. Top cart

2- When shopping for a new car, a professor was surprised to learn that a certain small compact car (mass=800 kg) will speed up (accelerate) at a faster rate than a large SUV (mass=2200 kg), although the SUV has the more powerful engine.

What is the reason that the compact car can speed up at a faster rate than the SUV, despite having a less powerful engine?

Select one:

a. The quantity pushing force / mass is greater for the compact car.

b. The quantity pushing force / mass is smaller for the compact car.

c. The quantity pushing force * mass is smaller for the compact car.

d. The quantity pushing force * mass is greater for the compact car.

3- Two skateboarders start themselves moving at the same speed side-by-side. After their initial pushes, they slow down gradually due to the frictional force acting on each of them. The masses of the skateboarders and the strengths of the frictional forces are shown in the force diagrams below.

Assuming they are initially moving at the same speed, which of the skateboarders will slow to a stop first, and why? (Hint: Think carefully about whether the first one to stop will have a higher or lower rate of change in his speed.)

Select one:

a. Skateboarder 1 will stop first because there is a stronger force acting opposite his motion.

b. Skateboarder 1 will stop first because he has more mass.

c. Skateboarder 2 will stop first because the quantity force strength/mass is higher for him.

d. Skateboarder 1 will stop first because the quantity force strength/mass is lower for him.

4- At the beginning of this extension activity, we asked how we could tell which of the two boxes below would speed up at the higher rate. The ideas you have developed in this assignment should help you decide the answer to this question.

Which of these two boxes will speed up at a higher rate, and why?

Select one:

a. The 50 kg box will speed up at a higher rate because the quantity net force strength/mass is greater for this box.

b. The 35 kg box will speed up at a higher rate because it has less mass.

c. The 35 kg box will speed up at a higher rate because the quantity net force strength/mass is greater for this box.

d. The 50 kg box will speed up at a higher rate because the net force acting on it is stronger.

5- Frank and Hector are moving two boxes across a rough floor. Frank pushes a 20 kg box with a force of 85 N while a frictional force of 75 N acts on it in the opposite direction. Meanwhile, Hector pushes on the 30 kg box with a force of 105 N while a frictional force of unknown strength acts on it.

Frank and Hector find that as their boxes are moving, they remain side-by-side, so they realize they must both be changing speed at the same rate. What is the strength of the frictional force acting on the 30 kg box?

Select one:

a. 90 N

b. 95 N

c. 75 N

d. 93 N

6- After completing the first part of this homework assignment, Luisa made this comment:

Which of the following statements best addresses Luisa’s concern?

Select one:

a. Another push is needed to start such an object moving, but during that push, the forces are still balanced.

b. Luisa is correct. An object with balanced forces cannot be moving.

c. Another push is needed to start such an object moving, and during that push, the forces are unbalanced.

7- Previously, you saw a demonstration (or a movie of one) involving a cart that had two fan units pushing on it in opposite directions with equal strength. With the cart initially at rest, it was given a push to start it moving. Here is the force diagram for that cart when it is at rest.

Which one of these force diagrams is the most appropriate for the cart at a moment during the short period it is getting the push that starts it moving?

Select one:

a. The bottom diagram

b. The middle diagram

c. The top diagram

8- This question again refers to the demonstration you previously saw that was referred to in the last question.

Assuming the speed of the cart is indeed constant, which one of these force diagrams is the most appropriate for the cart just after the initial push?

Select one:

a. The top diagram

b. The bottom diagram

c. The middle diagram

9- After a hockey player gives a puck a quick ‘hit’ with his stick, the puck slides across the ice at a speed that is essentially constant. A student in a previous class offered the following force diagram and explanation for why the speed of the puck is constant after the hit.

What is your evaluation of this explanation?

“The initial hit exerts a force on the puck, making the forces acting on the puck unbalanced, and so it starts to move. After the hit ends there is no net force acting on the puck, and when the net force acting on an object is zero, its speed does not change. So after the stick has lost contact with the puck, its speed remains constant.”

Select one:

a. Both the force diagram and the narrative are problematic.

b. The force diagram is problematic but the narrative is accurate.

c. Both the force diagram and the narrative are accurate.

d. The force diagram is accurate but the narrative is problematic.

10- At the beginning of this extension activity, we asked how we could tell which of the two boxes below would speed up at the higher rate. The ideas you have developed in this assignment should help you decide the answer to this question.

Which of these two boxes will speed up at a higher rate, and why?

Select one:

a. The 35 kg box will speed up at a higher rate because the quantity net force strength/mass is greater for this box.

b. The 50 kg box will speed up at a higher rate because the net force acting on it is stronger.

c. The 50 kg box will speed up at a higher rate because the quantity net force strength/mass is greater for this box.

d. The 35 kg box will speed up at a higher rate because it has less mass.

11- Frank and Hector are moving two boxes across a rough floor. Frank pushes a 20 kg box with a force of 85 N while a frictional force of 75 N acts on it in the opposite direction. Meanwhile, Hector pushes on the 30 kg box with a force of 105 N while a frictional force of unknown strength acts on it.

Frank and Hector find that as their boxes are moving, they remain side-by-side, so they realize they must both be changing speed at the same rate. What is the strength of the frictional force acting on the 30 kg box?

Select one:

a. 93 N

b. 75 N

c. 90 N

d. 95 N

12- In a question earlier in this extension, you considered the situation of moving a couch at a constant speed. You concluded that to move the couch at a constant speed, you would need to push it with a force that exactly balances the frictional force of 50 N acting on the couch.

Now consider a situation in which the couch is at rest. You push on the couch with a force of 25 N--and the couch still does not move. Which one of the force diagrams below best describes this situation?

Select one:

a. Choice a

b. Choice b

c. Choice c

13- Several children of different ages and sizes are playing a “tug-of-war” game over a large box of toys. The force diagrams below indicate the forces that between one and four children are exerting on the box at different times. The friction force that the floor exerts on the box is also shown. In each case, the box is moving to the right (as indicated by the red speed arrow).

In which example is the speed of the box not constant?

Select one:

a. Box A

b. Box B

c. Box C

d. Box D

14- Consider the force diagrams for four low-friction carts shown below. Each cart is subject to pushes from two fans (not shown) mounted on opposite ends of the cart. The forces the fans exert of the carts are shown. All four carts are moving to the right (the speed arrows are not shown). In each case, the cart experiences a small friction force in the opposite direction to its motion. These friction forces are also shown.

Each cart has a mass of 1 kg, but three of the carts also carry objects that contribute to their total mass. The total mass of each cart and the object it carries (if any) is indicated in the force diagrams below.

Which cart experiences the greatest rate of change in speed?

Select one:

a. Cart A

b. Cart B

c. Cart C

d. Cart D

15- Imagine an empty elevator is making a trip down from the 10th floor to the 1st floor of a building. The following three questions concern various stages in this journey.

The elevator starts at rest on the 10th floor and then starts to move downward, speeding up as it does so. While this is happening, how would the forces acting on the elevator compare?

Select one:

a. The upward force of the cable would be stronger than the downward gravitational force.

b. The upward force of the cable would be equal in strength to the downward gravitational force.

c. The upward force of the cable would be weaker than the downward gravitational force.

16- As it passes the 9th floor, the elevator reaches the desired descent speed and so travels from here down to the 2nd floor at a constant speed. While it is moving downward at a constant speed, how would the forces acting on the elevator compare?

Select one:

a. The upward force of the cable would be equal in strength to the downward gravitational force.

b. The upward force of the cable would be stronger than the downward gravitational force.

c. The upward force of the cable would be weaker than the downward gravitational force.

17- As the elevator passes the second floor, it starts to slow down so that it can stop at the first floor. While it is slowing down as it descends, how would the forces acting on the elevator compare?

Select one:

a. The upward force of the cable would be equal in strength to the downward gravitational force.

b. The upward force of the cable would be stronger than the downward gravitational force.

c. The upward force of the cable would be weaker than the downward gravitational force.

18- You have seen that the effect of the drag force on a skydiver is to make him speed up at a slower and slower rate as he falls, until he reaches his terminal velocity. Which of the speed-time graphs below shows this behavior?

Select one:

a. Graph A

b. Graph B

c. Graph C