Q: A 5 kg box is pulled across a frictionless surface with a constant force of 20 N. What is the acceleration of the box?

$$a = \frac{F}{m} = \frac{20}{5} = 4\ \text{m/s}^2$$

Q: A 2 kg object is hanging from a massless rope. What is the tension in the rope if the object is at rest?

$$T = mg = 2 \times 9.8 = 19.6\ \text{N}$$

Q: A 2.0 kg purse is dropped from a height of 45 m and reaches the ground at 25 m/s. What is the average force of air resistance during the fall? ($$g = 9.8\ \text{m/s}^2)$$

$$F_{\text{air}} = mg - m a = 2.0 \times 9.8 - 2.0 \times \frac{25^2$}{2 \times 45} = 19.6 - 13.9 = 5.7\ \text{N}$$

Q: A person stands on a scale in an elevator accelerating upward at $$2\ \text{m/s}^2. If $$the person's mass is $$60\ \text{kg}$$, what does the scale read?

$$F_N = m(g + a) = 60 \times (9.8 + 2) = 708\ \text{N}$$

Question 1

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

Accepted Answer

An object will remain at rest or move with constant velocity unless acted upon by a net external force.

$$\vec{F}_{	ext{net}} = 0 \implies \vec{v} = 	ext{constant}$$

Question 2

A 5 kg box is pulled across a frictionless surface with a constant force of 20 N. What is the acceleration of the box?

Accepted Answer

$$a = \frac{F}{m} = \frac{20}{5} = 4\ 	ext{m/s}^2$$

Question 3

Which of the following is NOT a contact force?

Accepted Answer

Gravitational force

Question 4

A 2 kg object is hanging from a massless rope. What is the tension in the rope if the object is at rest?

Accepted Answer

$$T = mg = 2 	imes 9.8 = 19.6\ 	ext{N}$$

Question 5

Two blocks, $$m_1 = 2.3$$\ 	ext{kg}$$ and m_2$ = 1.2\ 	ext{kg}$$, are pushed together on a frictionless surface by a force $$F = 3.2\ 	ext{N}. $$What is the acceleration of the system?

Accepted Answer

$$a = \frac{F}{m_1$ + m_2$} = \frac{3.2}{2.3 + 1.2} = 0.91\ 	ext{m/s}^2$$

Question 6

A 3.70 kg block ($$m_1) $$sits on a frictionless $$30^\circ$$ incline, connected by a massless cord over a pulley to a 2.30 kg hanging block ($$m_2). $$What is the acceleration of the system? ($$g = 10\ 	ext{m/s}^2)$$

Accepted Answer

$$a = \frac{m_2g - m_1g\sin 30^\circ}{m_1$ + m_2$} = \frac{2.30 	imes 10 - 3.70 	imes 10 	imes 0.5}{3.70 + 2.30} = 0.75\ 	ext{m/s}^2$$

Question 7

Which of the following best describes Newton's Third Law?

Accepted Answer

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

$$\vec{F}_{AB} = -\vec{F}_{BA}$$

Question 8

A 2.0 kg purse is dropped from a height of 45 m and reaches the ground at 25 m/s. What is the average force of air resistance during the fall? ($$g = 9.8\ 	ext{m/s}^2)$$

Accepted Answer

$$F_{	ext{air}} = mg - m a = 2.0 	imes 9.8 - 2.0 	imes \frac{25^2$}{2 	imes 45} = 19.6 - 13.9 = 5.7\ 	ext{N}$$

Question 9

Which of the following is the correct SI unit for force?

Accepted Answer

Newton (N)

Question 10

A person stands on a scale in an elevator accelerating upward at $$2\ 	ext{m/s}^2. If $$the person's mass is $$60\ 	ext{kg}$$, what does the scale read?

Accepted Answer

$$F_N = m(g + a) = 60 	imes (9.8 + 2) = 708\ 	ext{N}$$

Question 11

Which of the following is a correct statement about free body diagrams (FBDs)?

Accepted Answer

Only forces acting ON the object are included in the FBD.

Question 12

A dog is held by a leash at an angle. The tension in the leash can be decomposed into which two components?

Accepted Answer

A component parallel to the ground and a component perpendicular to the ground.

Question 13

A truck is pulled by a person using a rope. According to Newton's Third Law, which statement is correct?

Accepted Answer

The force the person exerts on the truck is equal in magnitude and opposite in direction to the force the truck exerts on the person.

Question 14

A block of mass $$m is at $$rest on a horizontal frictionless surface. If a constant force $$F is $$applied, what is the velocity after time $$t$$?

Accepted Answer

$$v = \frac{F}{m} t$$

Question 15

Which of the following is a non-inertial reference frame?

Accepted Answer

A car accelerating around a curve.

Question 16

A block of mass $$m_1 on a $$frictionless table is pushed by a force $$F$$ and is in contact with block $$m_2. $$What is the force that $$m_1$$ exerts on $$m_2$$?

Accepted Answer

$$F_{12} = \frac{m_2$}{m_1$ + m_2$} F$$

Question 17

Which of the following is the correct expression for the weight of an object of mass $$m$$ near the Earth's surface?

Accepted Answer

$$W = mg$$

Question 18

A 10 kg box is suspended by a rope. If the box is accelerating upward at $$2\ 	ext{m/s}^2$$, what is the tension in the rope?

Accepted Answer

$$T = m(g + a) = 10 	imes (9.8 + 2) = 118\ 	ext{N}$$

Newton’s Laws of Motion and Forces: Structured Study Notes

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