Q: A 1.0 kg block is pulled with a constant force and accelerates at $$3.0\ \mathrm{m/s^2$}. If a $$second block is pulled with the same force and accelerates at $$5.0\ \mathrm{m/s^2$}$$, what is the mass of the second block?

$$m = \frac{3.0\ \mathrm{m/s^2$}}{5.0\ \mathrm{m/s^2$}} \times 1.0\ \mathrm{kg} = 0.60\ \mathrm{kg}$$

Question 1

A sled is pushed across three different surfaces: smooth snow, slick ice, and a frictionless surface. On which surface will the sled continue moving at a constant velocity after being pushed, and why?

Accepted Answer

On the frictionless surface, because there is no force opposing its motion.

Question 2

A car crashes into a wall and comes to a sudden stop. The crash test dummy inside continues moving forward until it hits the dashboard. Which law of motion best explains the dummy's continued motion?

Accepted Answer

Newton's First Law: An object in motion remains in motion unless acted on by a net force.

Question 3

A spacecraft like Voyager is moving through deep space, far from any significant gravitational forces. What will happen to its motion if no forces act on it?

Accepted Answer

It will continue moving in a straight line at constant speed.

Question 4

Which of the following is a correct statement about the direction of the normal force?

Accepted Answer

The normal force is always perpendicular to the surface.

Question 5

A 2 kg block is pulled with a net force of 10 N. What is its acceleration?

Accepted Answer

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

Question 6

A block is at rest on a table. Which forces act on the block?

Accepted Answer

Gravity (weight) downward and normal force upward.

Question 7

A skier is being pulled up a hill at constant speed by a rope. Which statement is true about the forces acting on the skier?

Accepted Answer

The net force on the skier is zero.

Question 8

A ball is thrown straight up. Just after it leaves the thrower's hand (ignoring air resistance), what forces act on it?

Accepted Answer

Only gravity acts downward.

Question 9

A 1.0 kg block is pulled with a constant force and accelerates at $$3.0\ \mathrm{m/s^2$}. If a $$second block is pulled with the same force and accelerates at $$5.0\ \mathrm{m/s^2$}$$, what is the mass of the second block?

Accepted Answer

$$m = \frac{3.0\ \mathrm{m/s^2$}}{5.0\ \mathrm{m/s^2$}} 	imes 1.0\ \mathrm{kg} = 0.60\ \mathrm{kg}$$

Question 10

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

Accepted Answer

The skier, because she is accelerating.

Question 11

A ball is hanging from a string and is at rest. Which of the following statements is correct?

Accepted Answer

The tension in the string equals the weight of the ball.

Question 12

A person pushes a box across a table at a constant speed. Which statement best describes the forces acting on the box?

Accepted Answer

The pushing force is equal in magnitude and opposite in direction to the friction force.

Question 13

A rocket launches upward by expelling exhaust gases downward. According to Newton's third law, what is the action-reaction pair?

Accepted Answer

The rocket pushes the exhaust gases down; the exhaust gases push the rocket up.

Question 14

A mosquito collides head-on with a truck. During the collision, which statement is true about the forces they exert on each other?

Accepted Answer

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

Question 15

A 10 kg object is acted on by a net force of 50 N. What is its acceleration?

Accepted Answer

$$a = \frac{F}{m} = \frac{50\ \mathrm{N}}{10\ \mathrm{kg}} = 5\ \mathrm{m/s^2$}$$

Question 16

A block is pulled by two ropes at right angles to each other, with forces $$\vec{F}_1$$ and $$\vec{F}_2. $$What is the magnitude of the net force?

Accepted Answer

$$F_{net} = \sqrt{F_1$^2 + F_2$^2}$$

Question 17

A 51,000 kg airplane accelerates from rest to 70 m/s over a distance of 940 m. What is the net force required to achieve this acceleration?

Accepted Answer

$$F = ma = 51,000\ \mathrm{kg} 	imes 2.61\ \mathrm{m/s^2$} = 133,000\ \mathrm{N}$$

Question 18

A person stands on a scale in an elevator that is accelerating upward. How does the normal force (the scale reading) compare to the person's weight?

Accepted Answer

The normal force is greater than the person's weight.

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

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