Q: A 50 kg girl stands on a scale. What is the normal force exerted by the scale?

$$F_N = 50 \times 9.8 = 490\ \mathrm{N}$$

Q: A 600 kg car accelerates to the right at $$0.5\ \mathrm{m/s^2$}. If $$the force of friction is $$1000\ \mathrm{N}$$, what force is the engine providing?

$$F = ma + F_{friction} = 600 \times 0.5 + 1000 = 1300\ \mathrm{N}$$

Q: A 20 kg object weighs how much on Earth?

$$W = 20 \times 9.8 = 196\ \mathrm{N}$$

Question 1

A 40 N force pushes a 20 kg object to the left on a frictionless surface. What is the object's acceleration?

Accepted Answer

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

Question 2

Which of the following best describes the relationship between mass and weight?

Accepted Answer

Weight is the force of gravity acting on an object's mass.

Question 3

A 5 kg object is pushed right by a 20 N force and pulled right by a 10 N force. What is its acceleration?

Accepted Answer

$$a = \frac{20 + 10}{5} = 6\ \mathrm{m/s^2$}$$

Question 4

If the mass of an object is halved, what happens to its weight (assuming it remains on Earth)?

Accepted Answer

The weight is halved.

Question 5

A 50 kg girl stands on a scale. What is the normal force exerted by the scale?

Accepted Answer

$$F_N = 50 	imes 9.8 = 490\ \mathrm{N}$$

Question 6

When Sanjay pushes a refrigerator across a kitchen floor at a constant speed, the force of friction is:

Accepted Answer

Equal and opposite to Sanjay's push.

Question 7

A 600 kg car accelerates to the right at $$0.5\ \mathrm{m/s^2$}. If $$the force of friction is $$1000\ \mathrm{N}$$, what force is the engine providing?

Accepted Answer

$$F = ma + F_{friction} = 600 	imes 0.5 + 1000 = 1300\ \mathrm{N}$$

Question 8

Which of the following is NOT a correct statement of Newton's Laws?

Accepted Answer

An object in motion will eventually stop unless acted on by a force.

Question 9

A 20 kg object weighs how much on Earth?

Accepted Answer

$$W = 20 	imes 9.8 = 196\ \mathrm{N}$$

Question 10

If the net force on a cart is doubled but its acceleration does not change, what is a possible explanation?

Accepted Answer

The mass of the cart doubled when the force doubled.

Question 11

Which of the following best illustrates Newton's Third Law in a real-world scenario?

Accepted Answer

A swimmer pushes water backward and moves forward.

Question 12

A 20 kg box is pulled to the right with a force of 50 N. If friction opposes the motion with 10 N, what is the acceleration?

Accepted Answer

$$a = \frac{50 - 10}{20} = 2\ \mathrm{m/s^2$}$$

Question 13

Which of the following is true about mass and weight on the Moon compared to Earth?

Accepted Answer

Weight is less on the Moon, but mass is the same.

Question 14

A 3 kg object is accelerated at $$7\ \mathrm{m/s^2$}. $$What is the required net force?

Accepted Answer

$$F = 3 	imes 7 = 21\ \mathrm{N}$$

Question 15

When the string holding a ball is pulled down quickly and the bottom string breaks, this best illustrates the:

Accepted Answer

Mass of the ball.

Question 16

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

Accepted Answer

An object at rest remains at rest unless acted on by a net force.

Question 17

A 40 N object is weighed on Earth. What is its mass?

Accepted Answer

$$m = \frac{40}{9.8} \approx 4.08\ \mathrm{kg}$$

Question 18

If you push on a wall, according to Newton's Third Law, the wall:

Accepted Answer

Pushes back on you with an equal and opposite force.

Question 19

Which of the following is a property of friction?

Accepted Answer

It depends on the types of materials and how much they are pressed together.

Newton's Laws of Motion: Forces, Mass, Weight, and Friction

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