Practice Static Friction in Physics

Use these practice problems to test your method after reviewing the concept explanation and worked examples.

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Quick Recap

The friction force that prevents a stationary object from beginning to slide when an external force is applied, adjusting in magnitude up to a maximum.

It takes more force to get something moving than to keep it moving.

Showing a random 20 of 50 problems.

Example 1

hard
A 2020 kg block sits on level ground, μs=0.5\mu_s = 0.5, g=10g = 10 m/s2^2. A rope pulls at θ\theta above horizontal. Find the angle that minimizes the pulling force needed to set it in motion.

Example 2

medium
A 55 kg block on a 30∘30^\circ incline, μs=0.4\mu_s = 0.4, g=10g = 10 m/s2^2. Find the friction force needed to keep it static, and whether it actually does. (sin⁡30∘=0.5\sin 30^\circ = 0.5, cos⁡30∘=0.866\cos 30^\circ = 0.866.)

Example 3

medium
A 22 kg block sits on a board. The board is tilted slowly. Slipping begins at θ=18∘\theta = 18^\circ. Find μs\mu_s.

Example 4

hard
Two blocks AA (22 kg, top) and BB (33 kg, bottom) sit stacked on a frictionless floor. Between them μs=0.4\mu_s = 0.4. A horizontal force FF is applied to BB. Find the largest FF for which AA does not slide. (g=10g = 10 m/s2^2.)

Example 5

medium
A 2 kg book is pressed against a vertical wall by a horizontal force N=50 NN = 50\,\text{N}. If μs=0.5\mu_s = 0.5, g=10 m/s2g = 10\,\text{m/s}^2, will it stay (not slide down)?

Example 6

challenge
A 4 kg block rests on a 30∘30^\circ incline with μs=0.3\mu_s = 0.3, g=10 m/s2g = 10\,\text{m/s}^2 (sin⁡30∘=0.5\sin 30^\circ = 0.5, cos⁡30∘=0.866\cos 30^\circ = 0.866). It does not stay by friction alone. Find the minimum force along the incline (up-slope) to hold it static.

Example 7

easy
A 1010 kg block on a level floor has μs=0.4\mu_s = 0.4, g=10g = 10 m/s2^2. A force of 3535 N is applied horizontally. Find the static friction.

Example 8

hard
A 22 kg block sits on top of a 33 kg block; both on a frictionless floor. The contact coefficient is μs=0.2\mu_s = 0.2, g=10g = 10 m/s2^2. A horizontal force is applied to the top block. Find the largest force without slipping.

Example 9

medium
A car on a flat road accelerates from rest by static friction with the road. If μs=0.7\mu_s = 0.7 and g=10g = 10 m/s2^2, find the maximum acceleration.

Example 10

medium
A block needs at least 12 N12\,\text{N} to start moving and only 8 N8\,\text{N} to keep moving at constant speed, with N=40 NN = 40\,\text{N}. Find μs\mu_s and μk\mu_k.

Example 11

medium
A 10 kg block on a 20∘20^\circ incline, μs=0.5\mu_s = 0.5, g=10 m/s2g = 10\,\text{m/s}^2 (sin⁡20∘=0.342\sin 20^\circ = 0.342, cos⁡20∘=0.940\cos 20^\circ = 0.940). Does it stay at rest?

Example 12

hard
A car rounds a flat curve of radius 5050 m. μs=0.6\mu_s = 0.6, g=10g = 10 m/s2^2. Find the maximum speed without slipping.

Example 13

medium
A 66 kg block on level ground with μs=0.3\mu_s = 0.3, g=10g = 10 m/s2^2. A rope pulls at θ=37∘\theta = 37^\circ above horizontal with F=25F = 25 N. Does the block move? (sin⁡37∘=0.6\sin 37^\circ = 0.6, cos⁡37∘=0.8\cos 37^\circ = 0.8.)

Example 14

easy
A 55 kg block on a level surface barely slips when pushed with 3030 N. Find μs\mu_s. (g=10g = 10 m/s2^2.)

Example 15

medium
On a level surface, max static friction on a block is 1414 N, kinetic friction is 1010 N. A constant 1212 N push is applied. Does the block accelerate?

Example 16

medium
On a flat bed of a truck, a 5 kg5\,\text{kg} crate has μs=0.4\mu_s = 0.4, g=10 m/s2g = 10\,\text{m/s}^2. What maximum acceleration can the truck have without the crate sliding?

Example 17

challenge
A car on a banked curve of radius 8080 m, banking angle 20∘20^\circ, μs=0.3\mu_s = 0.3, g=10g = 10 m/s2^2 (sin⁡=0.342\sin = 0.342, cos⁡=0.940\cos = 0.940). Find the maximum speed without slipping.

Example 18

easy
A horizontal push of 88 N is applied to a 44 kg block on level ground. If μs=0.3\mu_s = 0.3 and g=10g = 10 m/s2^2, find the static friction.

Example 19

easy
Why is it generally harder to start a heavy box moving than to keep it sliding?

Example 20

easy
A resting crate has maximum static friction 50 N50\,\text{N}. A worker pushes with 30 N30\,\text{N}. What friction force acts on the crate?
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