Q: A 10 kg box rests on a horizontal steel surface. What is the maximum static friction force that can act on the box before it starts to move? (Use $$\mu_s$$ for steel on steel from the table and $$g = 9.8 \ \mathrm{m/s^2$}.)$$

$$f_{s,\text{max}} = \mu_s n = 0.74 \times 10 \times 9.8 = 72.52 \ \mathrm{N}$$

Q: A 5 kg block is sliding on a dry concrete floor. What is the kinetic friction force acting on the block? (Use $$\mu_k$$ for rubber on concrete (dry) from the table and $$g = 9.8 \ \mathrm{m/s^2$}.)$$

$$f_k = \mu_k n = 0.8 \times 5 \times 9.8 = 39.2 \ \mathrm{N}$$

Q: A 2 kg glass object is at rest on a glass table. What is the minimum horizontal force required to start moving the object? (Use $$\mu_s$$ for glass on glass and $$g = 9.8 \ \mathrm{m/s^2$}.)$$

$$f_{s,\text{min}} = 0.94 \times 2 \times 9.8 = 18.424 \ \mathrm{N}$$

Q: A 20 kg crate is being pulled across a wet concrete floor. What is the kinetic friction force acting on the crate? (Use $$\mu_k$$ for rubber on concrete (wet) and $$g = 9.8 \ \mathrm{m/s^2$}.)$$

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

Q: If a 15 kg aluminum block is at rest on a steel surface, what is the maximum static friction force before it starts to move? (Use $$\mu_s$$ for aluminum on steel and $$g = 9.8 \ \mathrm{m/s^2$}.)$$

$$f_{s,\text{max}} = 0.61 \times 15 \times 9.8 = 89.67 \ \mathrm{N}$$

Q: A 12 kg copper block is sliding on a steel surface. What is the kinetic friction force acting on the block? (Use $$\mu_k$$ for copper on steel and $$g = 9.8 \ \mathrm{m/s^2$}.)$$

$$f_k = 0.36 \times 12 \times 9.8 = 42.336 \ \mathrm{N}$$

Q: A 7 kg block is pulled across a steel surface with a constant velocity. What is the magnitude of the pulling force? (Use $$\mu_k$$ for steel on steel and $$g = 9.8 \ \mathrm{m/s^2$}.)$$

$$F = 0.57 \times 7 \times 9.8 = 39.102 \ \mathrm{N}$$

Q: A 4 kg block is at rest on a horizontal surface. The coefficient of static friction is $$0.85. $$What is the maximum force that can be applied horizontally before the block starts to move? ($$g = 9.8 \ \mathrm{m/s^2$})$$

$$f_{s,\text{max}} = 0.85 \times 4 \times 9.8 = 33.32 \ \mathrm{N}$$

Q: A 6 kg block is sliding on a glass surface. What is the kinetic friction force acting on the block? (Use $$\mu_k$$ for glass on glass and $$g = 9.8 \ \mathrm{m/s^2$}.)$$

$$f_k = 0.53 \times 6 \times 9.8 = 31.164 \ \mathrm{N}$$

Question 1

A 10 kg box rests on a horizontal steel surface. What is the maximum static friction force that can act on the box before it starts to move? (Use $$\mu_s$$ for steel on steel from the table and $$g = 9.8 \ \mathrm{m/s^2$}.)$$

Accepted Answer

$$f_{s,	ext{max}} = \mu_s n = 0.74 	imes 10 	imes 9.8 = 72.52 \ \mathrm{N}$$

Question 2

A 5 kg block is sliding on a dry concrete floor. What is the kinetic friction force acting on the block? (Use $$\mu_k$$ for rubber on concrete (dry) from the table and $$g = 9.8 \ \mathrm{m/s^2$}.)$$

Accepted Answer

$$f_k = \mu_k n = 0.8 	imes 5 	imes 9.8 = 39.2 \ \mathrm{N}$$

Question 3

Which material pair has the highest coefficient of static friction according to the table?

Accepted Answer

Rubber on concrete (dry)

Question 4

A 2 kg glass object is at rest on a glass table. What is the minimum horizontal force required to start moving the object? (Use $$\mu_s$$ for glass on glass and $$g = 9.8 \ \mathrm{m/s^2$}.)$$

Accepted Answer

$$f_{s,	ext{min}} = 0.94 	imes 2 	imes 9.8 = 18.424 \ \mathrm{N}$$

Question 5

A 20 kg crate is being pulled across a wet concrete floor. What is the kinetic friction force acting on the crate? (Use $$\mu_k$$ for rubber on concrete (wet) and $$g = 9.8 \ \mathrm{m/s^2$}.)$$

Accepted Answer

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

Question 6

If a 15 kg aluminum block is at rest on a steel surface, what is the maximum static friction force before it starts to move? (Use $$\mu_s$$ for aluminum on steel and $$g = 9.8 \ \mathrm{m/s^2$}.)$$

Accepted Answer

$$f_{s,	ext{max}} = 0.61 	imes 15 	imes 9.8 = 89.67 \ \mathrm{N}$$

Question 7

A 12 kg copper block is sliding on a steel surface. What is the kinetic friction force acting on the block? (Use $$\mu_k$$ for copper on steel and $$g = 9.8 \ \mathrm{m/s^2$}.)$$

Accepted Answer

$$f_k = 0.36 	imes 12 	imes 9.8 = 42.336 \ \mathrm{N}$$

Question 8

Which of the following statements is true regarding the coefficients of friction for Teflon on Teflon?

Accepted Answer

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

Question 9

A 3 kg object is at rest on a horizontal surface. If the coefficient of static friction is $$0.68$$ and the coefficient of kinetic friction is $$0.53$$, what is the frictional force if a $$10 \ \mathrm{N}$$ horizontal force is applied and the object does not move? ($$g = 9.8 \ \mathrm{m/s^2$})$$

Accepted Answer

$$f_s = 10 \ \mathrm{N}$$

Question 10

A 7 kg block is pulled across a steel surface with a constant velocity. What is the magnitude of the pulling force? (Use $$\mu_k$$ for steel on steel and $$g = 9.8 \ \mathrm{m/s^2$}.)$$

Accepted Answer

$$F = 0.57 	imes 7 	imes 9.8 = 39.102 \ \mathrm{N}$$

Question 11

A 4 kg block is at rest on a horizontal surface. The coefficient of static friction is $$0.85. $$What is the maximum force that can be applied horizontally before the block starts to move? ($$g = 9.8 \ \mathrm{m/s^2$})$$

Accepted Answer

$$f_{s,	ext{max}} = 0.85 	imes 4 	imes 9.8 = 33.32 \ \mathrm{N}$$

Question 12

Which of the following material pairs would result in the least resistance to sliding motion?

Accepted Answer

Teflon on Teflon

Question 13

A 6 kg block is sliding on a glass surface. What is the kinetic friction force acting on the block? (Use $$\mu_k$$ for glass on glass and $$g = 9.8 \ \mathrm{m/s^2$}.)$$

Accepted Answer

$$f_k = 0.53 	imes 6 	imes 9.8 = 31.164 \ \mathrm{N}$$

Question 14

A 9 kg block is at rest on a horizontal surface. If the coefficient of static friction is $$0.04$$, what is the maximum static friction force? ($$g = 9.8 \ \mathrm{m/s^2$})$$

Accepted Answer

$$f_{s,	ext{max}} = 0.04 	imes 9 	imes 9.8 = 3.528 \ \mathrm{N}$$

Question 15

A 10 kg block is pulled across a steel surface with a force of $$70 \ \mathrm{N}. If $$the block moves at a constant velocity, what is the coefficient of kinetic friction? ($$g = 9.8 \ \mathrm{m/s^2$})$$

Accepted Answer

$$\mu_k = \frac{70}{10 	imes 9.8} = 0.714$$

Question 16

A 2 kg block is at rest on a horizontal surface. If the coefficient of static friction is $$0.68$$ and the coefficient of kinetic friction is $$0.53$$, what is the frictional force if a $$5 \ \mathrm{N}$$ horizontal force is applied and the object does not move? ($$g = 9.8 \ \mathrm{m/s^2$})$$

Accepted Answer

$$f_s = 5 \ \mathrm{N}$$

Physics 250: Motion, Forces, and Friction – Essential Formulas and Concepts

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