Q: A 50 kg crate rests on a horizontal floor. If the coefficient of static friction between the crate and the floor is $$\mu_s = 0.4$$, what is the minimum force required to start moving the crate? (Take $$g = 9.8\ \mathrm{m/s^2$})$$

$$F = 50 \times 9.8 \times 0.4 = 196\ \mathrm{N}$$

Q: A person pushes a 103.7 kg box across a floor with a coefficient of kinetic friction $$\mu_k = 0.37. $$What is the force of friction opposing the motion? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

$$F_f = 103.7 \times 9.8 \times 0.37 = 376.6\ \mathrm{N}$$

Q: A 10 kg block is at rest on a horizontal surface. The coefficient of static friction is $$\mu_s = 0.5. $$What is the maximum horizontal force that can be applied before the block starts to move? ($$g = 9.8\ \mathrm{m/s^2$})$$

$$F_{max} = 10 \times 9.8 \times 0.5 = 49\ \mathrm{N}$$

Q: A 20 kg suitcase is dragged across a hotel lobby floor with a coefficient of kinetic friction $$\mu_k = 0.25. $$What is the minimum force required to keep the suitcase moving at constant velocity? ($$g = 9.8\ \mathrm{m/s^2$})$$

$$F = 20 \times 9.8 \times 0.25 = 49\ \mathrm{N}$$

Question 1

Which of the following best describes the direction of the frictional force acting on an object sliding across a horizontal surface?

Accepted Answer

Friction acts opposite to the direction of the object's motion.

Question 2

A 50 kg crate rests on a horizontal floor. If the coefficient of static friction between the crate and the floor is $$\mu_s = 0.4$$, what is the minimum force required to start moving the crate? (Take $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$F = 50 	imes 9.8 	imes 0.4 = 196\ \mathrm{N}$$

Question 3

Which statement correctly compares static and kinetic friction?

Accepted Answer

The coefficient of static friction is usually higher than that of kinetic friction.

Question 4

A person pushes a 103.7 kg box across a floor with a coefficient of kinetic friction $$\mu_k = 0.37. $$What is the force of friction opposing the motion? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$F_f = 103.7 	imes 9.8 	imes 0.37 = 376.6\ \mathrm{N}$$

Question 5

If the normal force between two surfaces is doubled, what happens to the frictional force, assuming the coefficient of friction remains constant?

Accepted Answer

The frictional force is doubled.

Question 6

Which of the following real-life scenarios best illustrates the difference between static and kinetic friction?

Accepted Answer

Trying to push a heavy couch that is at rest, and then continuing to push it once it starts sliding.

Question 7

The coefficient of friction ($$\mu) is $$defined as:

Accepted Answer

The ratio of the frictional force to the normal force.

Question 8

A 10 kg block is at rest on a horizontal surface. The coefficient of static friction is $$\mu_s = 0.5. $$What is the maximum horizontal force that can be applied before the block starts to move? ($$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$F_{max} = 10 	imes 9.8 	imes 0.5 = 49\ \mathrm{N}$$

Question 9

Why is it generally easier to keep an object moving than to start it moving from rest?

Accepted Answer

Because the coefficient of kinetic friction is lower than the coefficient of static friction.

Question 10

A 20 kg suitcase is dragged across a hotel lobby floor with a coefficient of kinetic friction $$\mu_k = 0.25. $$What is the minimum force required to keep the suitcase moving at constant velocity? ($$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$F = 20 	imes 9.8 	imes 0.25 = 49\ \mathrm{N}$$

Question 11

If a box is sliding to the right, in which direction does the frictional force act?

Accepted Answer

To the left

Question 12

Which of the following would increase the frictional force between two surfaces?

Accepted Answer

Increasing the normal force

Question 13

A car is parked on a hill. Which type of friction prevents it from sliding down?

Accepted Answer

Static friction

Question 14

A 5 kg book rests on a table. If the coefficient of static friction is $$\mu_s = 0.6$$, what is the maximum frictional force that can act on the book? ($$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$F_{max} = 5 	imes 9.8 	imes 0.6 = 29.4\ \mathrm{N}$$

Question 15

Which of the following best explains why anti-lock braking systems (ABS) improve car safety on wet roads?

Accepted Answer

ABS keeps the tires from locking, maintaining static friction rather than kinetic friction.

Question 16

A 15 kg sled is pulled across snow with a coefficient of kinetic friction $$\mu_k = 0.1. $$What is the frictional force acting on the sled? ($$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$F_f = 15 	imes 9.8 	imes 0.1 = 14.7\ \mathrm{N}$$

Question 17

If the coefficient of friction between a tire and dry pavement is $$\mu = 0.8$$, what is the maximum possible frictional force for a 1000 kg car? ($$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$F_{max} = 1000 	imes 9.8 	imes 0.8 = 7840\ \mathrm{N}$$

Friction: Concepts, Types, and Applications

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