Q: A 20 kg block on a frictionless surface accelerates from rest to 30 m/s in 6 s. What is the magnitude of the applied force?

$$F = m a = 20 \times 5 = 100\ \mathrm{N}$$

Q: A 1000-kg rocket accelerates vertically upward. The engines provide 25,000 N of thrust, gravity exerts 10,000 N downward, and air resistance is 5,000 N downward. What is the rocket’s velocity after 20 s, starting from rest?

$$v = 20\ \mathrm{m/s^2$} \times 20\ \mathrm{s} = 400\ \mathrm{m/s}$$

Q: A 100-kg load of bricks is being lowered by a cable at 5 m/s and comes to rest in 2 s. What is the tension in the cable during this interval? (Take $$g = 9.8\ \mathrm{m/s^2$})$$

$$T = 100 \times (9.8 - 2.5) = 730\ \mathrm{N}$$

Q: The Mars Rover Perseverance weighed about 10,000 N on Earth and 3,790 N on Mars. What is the gravitational acceleration on Mars?

$$g_{Mars} = \frac{3790}{10000} \times 9.8 = 3.7\ \mathrm{m/s^2$}$$

Q: In an Atwood machine, a 6 kg block and a 4 kg block are connected by a string over a pulley. What is the acceleration of the system? (Take $$g = 9.8\ \mathrm{m/s^2$})$$

$$a = \frac{(6 - 4) \times 9.8}{6 + 4} = 1.96\ \mathrm{m/s^2$}$$

Question 1

A 10 kg block is pulled to the right by a force of 70 N and to the left by a force of 20 N. What is the acceleration of the block? (Assume right is positive.)

Accepted Answer

$$a = \frac{70 - 20}{10} = 5\ \mathrm{m/s^2$}$$

Question 2

Which of the following best describes Newton’s First Law?

Accepted Answer

An object remains at rest or in uniform motion unless acted upon by a net force.

Question 3

A 6 kg box is pushed to the right with 20 N and to the left with 20 N. What is its acceleration?

Accepted Answer

$$a = 0\ \mathrm{m/s^2$}$$

Question 4

Which of the following is NOT a force acting on a couch being pushed across a rough floor?

Accepted Answer

Tension

Question 5

A 20 kg block on a frictionless surface accelerates from rest to 30 m/s in 6 s. What is the magnitude of the applied force?

Accepted Answer

$$F = m a = 20 	imes 5 = 100\ \mathrm{N}$$

Question 6

A 3-kg bucket is pulled upwards by a cord with a tension of 35 N. What is the acceleration of the bucket? (Take $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$a = \frac{35 - 29.4}{3} = 1.9\ \mathrm{m/s^2$}$$

Question 7

A 1000-kg rocket accelerates vertically upward. The engines provide 25,000 N of thrust, gravity exerts 10,000 N downward, and air resistance is 5,000 N downward. What is the rocket’s velocity after 20 s, starting from rest?

Accepted Answer

$$v = 20\ \mathrm{m/s^2$} 	imes 20\ \mathrm{s} = 400\ \mathrm{m/s}$$

Question 8

Which of the following is NOT an action-reaction pair according to Newton’s Third Law?

Accepted Answer

Weight of the book & normal force on the book

Question 9

A 100-kg load of bricks is being lowered by a cable at 5 m/s and comes to rest in 2 s. What is the tension in the cable during this interval? (Take $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$T = 100 	imes (9.8 - 2.5) = 730\ \mathrm{N}$$

Question 10

The Mars Rover Perseverance weighed about 10,000 N on Earth and 3,790 N on Mars. What is the gravitational acceleration on Mars?

Accepted Answer

$$g_{Mars} = \frac{3790}{10000} 	imes 9.8 = 3.7\ \mathrm{m/s^2$}$$

Question 11

Two blocks of mass 3 kg and 5 kg are connected by a string. If the 5 kg block is pulled to the right with a force of 30 N on a frictionless surface, what is the acceleration of the system?

Accepted Answer

$$a = \frac{30}{3 + 5} = 3.75\ \mathrm{m/s^2$}$$

Question 12

In an Atwood machine, a 6 kg block and a 4 kg block are connected by a string over a pulley. What is the acceleration of the system? (Take $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$a = \frac{(6 - 4) 	imes 9.8}{6 + 4} = 1.96\ \mathrm{m/s^2$}$$

Question 13

A 2 kg physics textbook is pushed to the right with a force of 20 N across a flat table. The force of kinetic friction is 8 N. What is the acceleration of the textbook?

Accepted Answer

$$a = \frac{20 - 8}{2} = 6\ \mathrm{m/s^2$}$$

Question 14

Which of the following statements about mass and weight is correct?

Accepted Answer

Mass does not change with location, but weight does.

Question 15

A 5.1 kg block is pulled vertically by a string. If the tension is 70 N, what is the acceleration of the block? (Take $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$a = \frac{70 - 49.98}{5.1} = 3.9\ \mathrm{m/s^2$}$$

Question 16

A 10 kg box is pushed to the left with 30 N and to the right with 70 N, and accelerates to the left at $$6\ \mathrm{m/s^2$}. $$What is the other force acting on the box?

Accepted Answer

130 N

Question 17

Which of the following is the correct sequence for solving a force problem involving multiple objects connected by strings?

Accepted Answer

Draw FBDs for all objects, choose direction of +, write $$\Sigma F = ma$$ for each, solve for acceleration, then solve for tensions.

Newton's Laws of Motion: Structured Study Notes

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