Q: A 60 kg student stands on a bathroom scale in an elevator that is accelerating upward at $$2\ \mathrm{m/s^2$}. $$What does the scale read?

$$W = 60 \times (9.8 + 2) = 708\ \mathrm{N}$$

Question 1

A car is moving at a constant velocity on a straight, level road. Which of the following statements best describes the net force acting on the car?

Accepted Answer

The net force is zero because all forces acting on the car are balanced.

Question 2

A 2 kg block is pulled across a frictionless surface with a constant force of 10 N. What is the acceleration of the block? Use Newton's second law.

Accepted Answer

$$a = \frac{10}{2} = 5\ \mathrm{m/s^2$}$$

Question 3

A skydiver reaches terminal velocity during free fall. Which of the following is true about the forces acting on her?

Accepted Answer

The net force is zero.

Question 4

A 60 kg student stands on a bathroom scale in an elevator that is accelerating upward at $$2\ \mathrm{m/s^2$}. $$What does the scale read?

Accepted Answer

$$W = 60 	imes (9.8 + 2) = 708\ \mathrm{N}$$

Question 5

Which of the following is NOT a force discussed in this chapter?

Accepted Answer

Orthogonal force

Question 6

A ball is thrown straight up. Ignore air resistance. Which force acts on the ball just after it leaves the thrower's hand?

Accepted Answer

Only the weight force, acting downward.

Question 7

A block is pushed across a table at a steady speed. Which statement is true about the forces acting on the block?

Accepted Answer

The pushing force is equal to the friction force.

Question 8

A 1 kg object is acted on by a net force of 4 N. What is its acceleration?

Accepted Answer

$$a = 4\ \mathrm{m/s^2$}$$

Question 9

A steel beam hangs from a cable as a crane lifts the beam. What forces act on the beam?

Accepted Answer

Gravity and tension in the cable

Question 10

A 40-car train travels at 40 mph on a straight track, while a skier speeds up as she skis downhill. On which is the net force greater?

Accepted Answer

The skier

Question 11

A mosquito runs head-on into a truck. Which is true during the collision?

Accepted Answer

The mosquito exerts the same force on the truck as the truck exerts on the mosquito.

Question 12

A bobsledder pushes her sled across horizontal snow to get it going, then jumps in. After she jumps in, the sled gradually slows to a halt. What forces act on the sled just after she’s jumped in?

Accepted Answer

Gravity, a normal force, and kinetic friction

Question 13

A 5 kg box is pulled with a force of 20 N across a rough surface. If the friction force is 5 N, what is the acceleration of the box?

Accepted Answer

$$a = \frac{20 - 5}{5} = 3\ \mathrm{m/s^2$}$$

Question 14

Which of the following best describes Newton’s third law?

Accepted Answer

For every force, there is an equal and opposite force acting on a different object.

Question 15

A car is towed to the right at constant speed. Which statement is true about the net force acting on the car?

Accepted Answer

The net force is zero.

Question 16

A 10 kg object is acted on by two forces: 30 N to the right and 10 N to the left. What is the acceleration of the object?

Accepted Answer

$$a = \frac{30 - 10}{10} = 2\ \mathrm{m/s^2$}$$\ 	ext{to the right}

Question 17

A block is at rest on a horizontal surface. Which of the following statements is correct about the forces acting on the block?

Accepted Answer

The normal force is equal to the weight.

Question 18

A rocket expels gas molecules at high speed out the back. What is the direction of the thrust force on the rocket?

Accepted Answer

Opposite to the direction of the expelled gas.

Question 19

A 1 kg block is pulled with a constant force and accelerates at $$2\ \mathrm{m/s^2$}. If $$the same force is applied to a 2 kg block, what will be its acceleration?

Accepted Answer

$$1\ \mathrm{m/s^2$}$$

Chapter 4: Forces and Newton’s Laws of Motion – Study Notes

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