Q: A block is at rest on a table. The forces acting on it are its weight and the normal force. If the block's mass is 5 kg, what is the magnitude of the normal force?

$N = mg = 5 \times 9.8 = 49\ \mathrm{N}$

Q: A 50 kg skier slides down a frictionless slope inclined at $30^\circ$ to the horizontal. What is the acceleration of the skier?

$a = g \sin 30^\circ = 9.8 \times 0.5 = 4.9\ \mathrm{m/s^2}$

Q: A person stands on a scale in an elevator accelerating upward at $2\ \mathrm{m/s^2}$. If the person's mass is 70 kg, what is the apparent weight?

$N = m(g + a) = 70 \times (9.8 + 2) = 826\ \mathrm{N}$

Question 1

Which of the following best describes Newton's First Law of Motion?

Accepted Answer

An object at rest will remain at rest, and an object in motion will continue moving in a straight line at constant velocity unless acted upon by a net force.

Question 2

A 10 kg shopping cart is pushed with a net force of 30 N. What is its acceleration?

Accepted Answer

$a = \frac{30}{10} = 3\ \mathrm{m/s^2}$

Question 3

Which equation correctly expresses Newton's Second Law of Motion?

Accepted Answer

$\sum F = ma$

Question 4

A person kicks a soccer ball. According to Newton's Third Law, what happens?

Accepted Answer

The ball exerts an equal and opposite force on the person's foot.

Question 5

A block is at rest on a table. The forces acting on it are its weight and the normal force. If the block's mass is 5 kg, what is the magnitude of the normal force?

Accepted Answer

$N = mg = 5 	imes 9.8 = 49\ \mathrm{N}$

Question 6

A 20 kg crate is pulled horizontally with a force of 100 N. If the frictional force is 40 N, what is the acceleration of the crate?

Accepted Answer

$a = \frac{100 - 40}{20} = 3\ \mathrm{m/s^2}$

Question 7

A 50 kg skier slides down a frictionless slope inclined at $30^\circ$ to the horizontal. What is the acceleration of the skier?

Accepted Answer

$a = g \sin 30^\circ = 9.8 	imes 0.5 = 4.9\ \mathrm{m/s^2}$

Question 8

Which statement about mass and weight is correct?

Accepted Answer

Mass is the quantity of matter in an object, while weight is the force due to gravity acting on that mass.

Question 9

A rocket launches in space. Which law of motion explains the rocket's upward movement?

Accepted Answer

Newton's Third Law: The rocket pushes exhaust gases downward, and the gases push the rocket upward.

Question 10

A person stands on a scale in an elevator accelerating upward at $2\ \mathrm{m/s^2}$. If the person's mass is 70 kg, what is the apparent weight?

Accepted Answer

$N = m(g + a) = 70 	imes (9.8 + 2) = 826\ \mathrm{N}$

Question 11

A block slides down a rough inclined plane at constant velocity. What can you conclude about the forces acting on the block?

Accepted Answer

The net force along the incline is zero; friction balances the component of gravity down the slope.

Question 12

If the net force acting on an object is zero, which of the following is true?

Accepted Answer

The object is in equilibrium and will either remain at rest or move at constant velocity.

Question 13

A 4 kg object is acted upon by a net force of 12 N. What is its acceleration?

Accepted Answer

$a = \frac{12}{4} = 3\ \mathrm{m/s^2}$

Question 14

Which of the following is a correct statement of Newton's Third Law?

Accepted Answer

For every action, there is an equal and opposite reaction.

Question 15

A person pushes a 20 kg box with a force of 60 N. The box moves with an acceleration of $2\ \mathrm{m/s^2}$. What is the magnitude of the frictional force?

Accepted Answer

$f = F - ma = 60 - 20 	imes 2 = 20\ \mathrm{N}$

Question 16

A hammer strikes a nail and comes to a stop. Which law of motion explains why the hammer stops?

Accepted Answer

Newton's Third Law: The nail exerts an equal and opposite force on the hammer.

Question 17

A 100 g mass is placed on Earth. What is its weight?

Accepted Answer

$W = mg = 0.1 	imes 9.8 = 0.98\ \mathrm{N}$

Question 18

A person walks by pushing backward on the ground. What force propels the person forward?

Accepted Answer

The ground exerts an equal and opposite forward force on the person.

Newton's Laws of Motion: Concepts, Applications, and Examples

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