Q: A 1000-kg car accelerates at $$0.5g on a $$horizontal road. What is the net force required to produce this acceleration? ($$g = 9.8\ \mathrm{m/s^2$})$$

$$F = 1000 \times 0.5 \times 9.8 = 4900\ \mathrm{N}$$

Q: A 65-kg woman stands on a scale in an elevator accelerating downward at $$0.20g. $$What does the scale read?

$$F_N = m(g - 0.20g) = 65 \times 0.8 \times 9.8 = 509.6\ \mathrm{N}$$

Q: A 10.0-kg box is pulled along a horizontal surface with a force of $$40.0\ \mathrm{N} at a$$ $$30.0^\circ$$ angle above the horizontal. The coefficient of kinetic friction is $$\mu_k = 0.30. $$What is the acceleration of the box?

$$a = \frac{40.0 \cos 30.0^\circ - 0.30 \times (98.0 - 40.0 \sin 30.0^\circ)}{10.0} = 1.54\ \mathrm{m/s^2$}$$

Q: A 10.0-kg box rests on a horizontal floor. The coefficient of static friction is $$\mu_s = 0.40. $$What is the maximum force of static friction before the box starts to move?

$$F_{fr, max} = \mu_s F_N = 0.40 \times 98.0 = 39.2\ \mathrm{N}$$

Question 1

A 1000-kg car accelerates at $$0.5g on a $$horizontal road. What is the net force required to produce this acceleration? ($$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$F = 1000 	imes 0.5 	imes 9.8 = 4900\ \mathrm{N}$$

Question 2

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

Accepted Answer

An object will remain at rest or in uniform motion in a straight line unless acted upon by a net external force.

Question 3

A 10.0-kg box rests on a frictionless table. If a force of $$40.0\ \mathrm{N} is $$applied at a $$30.0^\circ$$ angle above the horizontal, what is the acceleration of the box?

Accepted Answer

$$a = \frac{40.0 \cos 30.0^\circ}{10.0} = 3.46\ \mathrm{m/s^2$}$$

Question 4

A 65-kg woman stands on a scale in an elevator accelerating downward at $$0.20g. $$What does the scale read?

Accepted Answer

$$F_N = m(g - 0.20g) = 65 	imes 0.8 	imes 9.8 = 509.6\ \mathrm{N}$$

Question 5

Which of the following is NOT an inertial reference frame?

Accepted Answer

A car turning around a curve at constant speed.

Question 6

A 10.0-kg box is pulled along a horizontal surface with a force of $$40.0\ \mathrm{N} at a$$ $$30.0^\circ$$ angle above the horizontal. The coefficient of kinetic friction is $$\mu_k = 0.30. $$What is the acceleration of the box?

Accepted Answer

$$a = \frac{40.0 \cos 30.0^\circ - 0.30 	imes (98.0 - 40.0 \sin 30.0^\circ)}{10.0} = 1.54\ \mathrm{m/s^2$}$$

Question 7

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

Accepted Answer

Mass is a measure of inertia; weight is the force of gravity acting on an object.

Question 8

A 10.0-kg box rests on a horizontal floor. The coefficient of static friction is $$\mu_s = 0.40. $$What is the maximum force of static friction before the box starts to move?

Accepted Answer

$$F_{fr, max} = \mu_s F_N = 0.40 	imes 98.0 = 39.2\ \mathrm{N}$$

Question 9

Two boxes, $$m_A = 10.0\ \mathrm{kg}$$ and $$m_B = 12.0\ \mathrm{kg}$$, are connected by a cord on a frictionless table. A force of $$40.0\ \mathrm{N} is $$applied to box A. What is the acceleration of the system?

Accepted Answer

$$a = \frac{40.0}{10.0 + 12.0} = 1.82\ \mathrm{m/s^2$}$$

Question 10

A rocket launches vertically. According to Newton's Third Law, what is the reaction force to the exhaust gases being expelled downward?

Accepted Answer

The upward force exerted on the rocket by the exhaust gases.

Question 11

A 10.0-kg box is at rest on a horizontal surface. If a horizontal force of $$20\ \mathrm{N} is $$applied and the coefficients of static and kinetic friction are $$\mu_s = 0.40$$ and $$\mu_k = 0.30$$, what is the friction force acting on the box?

Accepted Answer

$$F_{fr} = 20\ \mathrm{N} ($$static friction, since $$F_A \u003c F_{fr, max})$$

Question 12

A person pushes a book across a table at constant speed. Which statement is true about the forces acting on the book?

Accepted Answer

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

Question 13

A 1500-kg car traveling at $$100\ \mathrm{km/h}$$ must stop in $$55\ \mathrm{m}. $$What average net force is required to stop the car?

Accepted Answer

$$F = \frac{m v^2$}{2d} = \frac{1500 	imes (27.78)^2$}{2 	imes 55} = 10500\ \mathrm{N}$$

Question 14

A box is at rest on an incline. Which force is responsible for preventing the box from sliding down?

Accepted Answer

The static friction force.

Question 15

A person stands on a scale in an elevator moving downward at constant speed. What does the scale read?

Accepted Answer

The scale reads the true weight, $$mg.$$

Question 16

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

Accepted Answer

Newton ($$\mathrm{N})$$

Question 17

A 10.0-kg box is pulled with a force of $$40.0\ \mathrm{N} at a$$ $$30.0^\circ$$ angle above the horizontal. If the box is on a frictionless surface, what is the normal force exerted by the table?

Accepted Answer

$$F_N = mg - F_P \sin 30.0^\circ = 98.0 - 20.0 = 78.0\ \mathrm{N}$$

Newton’s Laws of Motion and Applications: Forces, Free-Body Diagrams, and Friction

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