Q: A box weighing $$100\ \mathrm{N} is at $$rest on a surface with $$\mu_s = 0.4. $$What is the maximum static friction force?

$$f_{s,\max} = \mu_s n = 0.4 \times 100\ \mathrm{N} = 40\ \mathrm{N}$$

Q: A 32 kg sofa is pushed at constant speed across a floor with $$\mu_k = 0.080. $$What force does Carol apply?

$$F = \mu_k mg = 0.080 \times 32\ \mathrm{kg} \times 9.8\ \mathrm{m/s^2$} = 25\ \mathrm{N}$$

Question 1

A 500 N orangutan hangs motionless from a vertical rope. What is the tension in the rope?

Accepted Answer

$$T = 500\ \mathrm{N}$$

Question 2

A car of mass $$1500\ \mathrm{kg} is $$towed at a steady speed by a rope at $$20^\circ$$ above the horizontal. If the friction force is $$320\ \mathrm{N}$$, what is the tension in the rope?

Accepted Answer

$$T = \dfrac{320\ \mathrm{N}}{\cos 20^\circ}$$

Question 3

Which of the following best describes static equilibrium?

Accepted Answer

An object at rest with $$\sum \vec{F} = 0$$

Question 4

A 24 kg lawn mower is pushed with a net force of $$51\ \mathrm{N}$$ parallel to the ground. What is its acceleration?

Accepted Answer

$$a = \dfrac{51\ \mathrm{N}}{24\ \mathrm{kg}} = 2.1\ \mathrm{m/s^2$}$$

Question 5

A 75 kg man stands on a scale in an elevator accelerating upward at $$1.2\ \mathrm{m/s^2$}. $$What does the scale read?

Accepted Answer

$$F_n = (75\ \mathrm{kg})(9.8\ \mathrm{m/s^2$} + 1.2\ \mathrm{m/s^2$}) = 825\ \mathrm{N}$$

Question 6

Which of the following is true about the normal force $$n$$ acting on an object at rest on a horizontal surface?

Accepted Answer

$$n = mg if no $$other vertical forces act

Question 7

A 1.2 kg book is pressed down with a $$15\ \mathrm{N}$$ force on a table. What is the normal force from the table?

Accepted Answer

$$n = 15\ \mathrm{N} + (1.2\ \mathrm{kg})(9.8\ \mathrm{m/s^2$}) = 27\ \mathrm{N}$$

Question 8

A skier slides down a $$27^\circ$$ frictionless slope. What is the skier’s acceleration?

Accepted Answer

$$a = g \sin 27^\circ$$

Question 9

A box weighing $$100\ \mathrm{N} is at $$rest on a surface with $$\mu_s = 0.4. $$What is the maximum static friction force?

Accepted Answer

$$f_{s,\max} = \mu_s n = 0.4 	imes 100\ \mathrm{N} = 40\ \mathrm{N}$$

Question 10

A box is pulled by a horizontal rope with tension $$T$$ and does not move. Which statement is correct?

Accepted Answer

The static friction force equals the tension: $$f_s = T$$

Question 11

A 32 kg sofa is pushed at constant speed across a floor with $$\mu_k = 0.080. $$What force does Carol apply?

Accepted Answer

$$F = \mu_k mg = 0.080 	imes 32\ \mathrm{kg} 	imes 9.8\ \mathrm{m/s^2$} = 25\ \mathrm{N}$$

Question 12

Which of the following is NOT a property of kinetic friction?

Accepted Answer

It increases as the speed of sliding increases

Question 13

A 5.0 kg block A is pushed with a 3.0 N force and is in contact with a 10 kg block B on a frictionless surface. What force does A exert on B?

Accepted Answer

$$F_{A\ on\ B} = \dfrac{3.0\ \mathrm{N}}{1 + 5.0/10} = 2.0\ \mathrm{N}$$

Question 14

Which statement about tension in a massless rope is correct?

Accepted Answer

The tension is the same at all points along the rope

Question 15

A 200 kg stage set is connected by a rope over a pulley to a 100 kg stagehand. When released, what is the acceleration of the system?

Accepted Answer

$$a = \dfrac{(200\ \mathrm{kg} - 100\ \mathrm{kg})g}{200\ \mathrm{kg} + 100\ \mathrm{kg}} = 3.3\ \mathrm{m/s^2$}$$

Question 16

A 50 kg student stands on a scale in a falling elevator (cable breaks). What does the scale read?

Accepted Answer

$$0\ \mathrm{N}$$

Question 17

Which of the following pairs is an action/reaction pair according to Newton’s third law?

Accepted Answer

The normal force on A due to B and the normal force on B due to A

Chapter 5: Applying Newton’s Laws – Equilibrium, Dynamics, Forces, and Friction

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