Q: A 70 kg person stands on a scale in an elevator. The scale reads 750 N as the elevator slows to a stop. What is the acceleration of the elevator?

$$a = \frac{750 - (70 \times 9.8)}{70} = 0.91\ \mathrm{m/s^2$}$$\ (upward)

Q: A 50 kg student stands in a 1000 kg elevator at rest. As the elevator accelerates upward, her apparent weight is 600 N. What is the acceleration of the elevator?

$$a = \frac{600 - (50 \times 9.8)}{50} = 0.4\ \mathrm{m/s^2$}$$\ (upward)

Question 1

A car with a mass of 1500 kg is being towed at a constant speed by a rope that makes a 20° angle with the horizontal. If the friction force opposing the motion is 320 N, what is the tension in the rope?

Accepted Answer

$$T = \frac{320}{\cos 20^\circ} = 340\ \mathrm{N}$$

Question 2

A 46 g golf ball is putted with an initial speed of 3.0 m/s. If friction exerts a 0.020 N retarding force, what is the acceleration of the ball?

Accepted Answer

$$a = \frac{-0.020}{0.046} = -0.43\ \mathrm{m/s^2$}$$

Question 3

Which of the following statements about Newton’s Third Law is correct?

Accepted Answer

Action/reaction forces act on different objects and are equal in magnitude but opposite in direction.

Question 4

A 70 kg person stands on a scale in an elevator. The scale reads 750 N as the elevator slows to a stop. What is the acceleration of the elevator?

Accepted Answer

$$a = \frac{750 - (70 	imes 9.8)}{70} = 0.91\ \mathrm{m/s^2$}$$\ (upward)

Question 5

A 500 N orangutan hangs motionless from a vertical rope. What is the tension in the rope?

Accepted Answer

$$T = 500\ \mathrm{N}$$

Question 6

A car is parked on a hill. Which force is responsible for preventing it from sliding down?

Accepted Answer

Static friction

Question 7

Which of the following best describes the condition for equilibrium in two dimensions?

Accepted Answer

$$\sum F_x = 0$$ and $$\sum F_y = 0$$

Question 8

A rocket launches by expelling hot gases backward. According to Newton’s third law, what is the force that propels the rocket forward?

Accepted Answer

The gases push forward on the rocket with a force equal in magnitude and opposite in direction to the force the rocket exerts on the gases.

Question 9

A 50 kg student stands in a 1000 kg elevator at rest. As the elevator accelerates upward, her apparent weight is 600 N. What is the acceleration of the elevator?

Accepted Answer

$$a = \frac{600 - (50 	imes 9.8)}{50} = 0.4\ \mathrm{m/s^2$}$$\ (upward)

Question 10

Which of the following is NOT a correct statement about free-body diagrams?

Accepted Answer

The net force vector is drawn on the particle itself.

Question 11

A car accelerates from rest to 12 m/s in 10 s while being towed by a rope at a 20° angle to the horizontal. The car’s mass is 1500 kg and the friction force is 320 N. What is the horizontal component of the acceleration?

Accepted Answer

$$a_x = \frac{12 - 0}{10} = 1.2\ \mathrm{m/s^2$}$$

Question 12

Which of the following is the correct expression for the weight of an object of mass $$m on $$Earth?

Accepted Answer

$$w = m g$$

Question 13

A person in free fall experiences which of the following?

Accepted Answer

Zero apparent weight

Question 14

A block is pulled across a horizontal surface at constant velocity. Which statement is true about the forces acting on the block?

Accepted Answer

The net force on the block is zero.

Question 15

Which of the following is a correct statement about mass and weight?

Accepted Answer

Mass is constant everywhere, but weight depends on the local gravitational field.

Question 16

A runner pushes backward on the ground with her foot. According to Newton’s third law, what is the reaction force?

Accepted Answer

The ground pushes forward on her foot.

Question 17

A block is at rest on a frictionless surface and is pulled by a string at an angle. Which force is responsible for the block not accelerating horizontally?

Accepted Answer

There is no horizontal force; only the vertical component of tension and the normal force balance the weight.

Study Guide: Forces and Newton’s Laws of Motion & Applying Newton’s Laws

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