Q: A $$5\ \mathrm{kg}$$ box is on an inclined plane at $$30^\circ to $$the horizontal. If $$\mu_k = 0.1$$, what is the frictional force acting on the box? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

$$0.1 \times 5 \times 9.8 \times \cos(30^\circ)$$

Q: A $$15\ \mathrm{kg}$$ box is pulled horizontally with a force of $$60\ \mathrm{N}. If$$ $$\mu_k = 0.2$$, what is the acceleration of the box? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

$$a = \frac{60 - 0.2 \times 15 \times 9.8}{15}$$

Question 1

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

Accepted Answer

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

Question 2

A box is at rest on a horizontal surface. Which type of friction prevents it from moving when a small force is applied?

Accepted Answer

Static friction

Question 3

Given the equation for static friction $$f \leq \mu_s N$$, what does $$\mu_s$$ represent?

Accepted Answer

The coefficient of static friction

Question 4

A 10 kg box is pulled across a floor with a coefficient of kinetic friction $$\mu_k = 0.3. $$What is the magnitude of the frictional force acting on the box? (Assume $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$29.4\ \mathrm{N}$$

Question 5

Which of the following statements is TRUE regarding the coefficients of friction?

Accepted Answer

The coefficient of static friction is always greater than the coefficient of kinetic friction.

Question 6

A force of $$40\ \mathrm{N} is $$applied at a $$30^\circ$$ angle above the horizontal to a $$10\ \mathrm{kg}$$ box on a floor with $$\mu_k = 0.3. $$What is the normal force acting on the box? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$98\ \mathrm{N} - 40\sin(30^\circ)$$

Question 7

If a box is moving at constant velocity across a surface, what can be said about the net force acting on it?

Accepted Answer

The net force is zero.

Question 8

A $$5\ \mathrm{kg}$$ box is on an inclined plane at $$30^\circ to $$the horizontal. If $$\mu_k = 0.1$$, what is the frictional force acting on the box? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$0.1 	imes 5 	imes 9.8 	imes \cos(30^\circ)$$

Question 9

Which of the following best explains why friction occurs between two surfaces?

Accepted Answer

Because of microscopic bumps and imperfections on the surfaces.

Question 10

A person pushes a box but it does not move. Which force is responsible for keeping the box stationary?

Accepted Answer

Static friction

Question 11

If the maximum static friction force for a box is $$20\ \mathrm{N}$$ and you apply a $$25\ \mathrm{N}$$ force, what happens?

Accepted Answer

The box starts moving and kinetic friction takes over.

Question 12

Which equation correctly represents the magnitude of kinetic friction?

Accepted Answer

$$f_k = \mu_k N$$

Question 13

A $$15\ \mathrm{kg}$$ box is pulled horizontally with a force of $$60\ \mathrm{N}. If$$ $$\mu_k = 0.2$$, what is the acceleration of the box? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$a = \frac{60 - 0.2 	imes 15 	imes 9.8}{15}$$

Question 14

Which of the following is NOT a factor that affects the magnitude of friction between two surfaces?

Accepted Answer

The area of contact

Question 15

Why is the force of friction considered a vector quantity?

Accepted Answer

Because it has both magnitude and direction.

Question 16

A $$20\ \mathrm{kg}$$ box is at rest on a floor with $$\mu_s = 0.4. $$What is the maximum static friction force that can act on the box? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$0.4 	imes 20 	imes 9.8$$

Question 17

When pulling a box at an angle, why must you consider both the vertical and horizontal components of the applied force?

Accepted Answer

Because the vertical component affects the normal force and thus the friction.

Question 18

Which real-world scenario best illustrates the concept of static friction?

Accepted Answer

A book resting on a table, not moving despite a gentle push.

Question 19

If the coefficient of kinetic friction between rubber and concrete is $$0.7$$, what is the frictional force on a $$50\ \mathrm{kg}$$ tire? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$0.7 	imes 50 	imes 9.8$$

Question 20

Which of the following best describes the transition from static to kinetic friction?

Accepted Answer

Kinetic friction takes over once the applied force exceeds maximum static friction.

Chapter 5: Friction and Circular Motion – Study Notes

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