Question 1

A block A with mass $$m$$ and block B with mass $$2m$$ are initially at rest in an elevator. The elevator begins to move downward with an acceleration $$a ($$where $$a \u003c g). $$What is the normal force exerted by the floor on block A?

Accepted Answer

$$N_A = m(g - a)$$

Question 2

Two balls, A and B, with masses $$m$$ and $$2m$$ respectively, are thrown upward with the same speed $$v. $$What is the ratio of the initial kinetic energy of ball B to that of ball A?

Accepted Answer

$$2:1$$

Question 3

When the balls reach their maximum height, what is the ratio of the kinetic energy of ball B to that of ball A?

Accepted Answer

$$0:0$$

Question 4

If the elevator is accelerating downward at $$a = g/2$$, what is the net force acting on block B of mass $$2m$$?

Accepted Answer

$$F_{net} = 2m a = 2m \frac{g}{2} = mg$$

Question 5

Consider the system consisting of both block A and block B in the elevator. What is the total normal force exerted by the floor on both blocks when the elevator is at rest?

Accepted Answer

$$N_{total} = 3mg$$

Question 6

A ball of mass $$m is $$thrown upward with speed $$v. $$What is the maximum height $$h it $$reaches?

Accepted Answer

$$h = \frac{v^2$}{2g}$$

Question 7

If block A is accelerating downward in the elevator, which force diagram correctly represents the forces acting on block A?

Accepted Answer

Upward normal force $$N_A$$ and downward gravitational force $$mg$$, with $$N_A \u003c mg$$

Question 8

For the system consisting of ball B and the Earth, what is the change in gravitational potential energy as the ball moves from $$y_0 to$$ $$y_1$$?

Accepted Answer

$$\Delta U = 2m g (y_1$ - y_0$)$$

Question 9

Which of the following is an example of an action-reaction pair according to Newton's third law?

Accepted Answer

The force of block A on the elevator floor and the force of the elevator floor on block A

Question 10

If the elevator is accelerating upward with acceleration $$a$$, what is the normal force on block B of mass $$2m$$?

Accepted Answer

$$N_B = 2m(g + a)$$

Question 11

A ball of mass $$m is $$thrown upward with speed $$v. At $$its highest point, what is its kinetic energy?

Accepted Answer

$$0$$

Question 12

If the elevator is at rest, what is the net external force on block A?

Accepted Answer

$$0$$

Question 13

Which system would you consider to analyze the change in kinetic energy of both balls and the Earth as the balls move upward?

Accepted Answer

System BE (balls and Earth)

Question 14

If block A and block B are both accelerating upward in the elevator, which of the following statements is true about the normal forces acting on them?

Accepted Answer

Both normal forces are greater than their respective weights

Question 15

A ball of mass $$m is $$thrown upward with speed $$v. $$What is the work done by gravity as the ball rises to its maximum height?

Accepted Answer

$$W = -mg h = -m g \frac{v^2$}{2g} = -\frac{1}{2} m v^2$

Question 16

If the elevator cable snaps and the elevator is in free fall, what is the normal force on block A?

Accepted Answer

$$N_A = 0$$

Question 17

Which of the following best describes the change in gravitational potential energy for ball A as it moves from $$y_0 to$$ $$y_1$$?

Accepted Answer

$$\Delta U = m g (y_1$ - y_0$)$$

Question 18

If the elevator is accelerating downward with acceleration $$a$$, which of the following is true about the net force on block A?

Accepted Answer

$$F_{net} = m a$$

Question 19

When analyzing the forces on block B, which force is NOT considered an external force acting on the system consisting of block B only?

Accepted Answer

The force from block A

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