Practice Kinetic 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 opposes sliding motion between two surfaces in contact; in the simple dry-friction model its magnitude is fk=μkNf_k = \mu_k N, directed opposite to the motion.

Once something is moving, friction resists its motion — less than static friction, but still present.

Showing a random 20 of 50 problems.

Example 1

easy
A 20 kg20\,\text{kg} box slides on a level floor with μk=0.2\mu_k = 0.2 and g=10 m/s2g = 10\,\text{m/s}^2. Find the kinetic friction force.

Example 2

hard
A 3 kg3\,\text{kg} block slides up a 30∘30^\circ incline at 8 m/s8\,\text{m/s} with μk=0.2\mu_k = 0.2 and g=10 m/s2g = 10\,\text{m/s}^2. How far up does it travel before momentarily stopping?

Example 3

easy
A 2 kg block slides on level ground, μk=0.5\mu_k = 0.5, g=10 m/s2g = 10\,\text{m/s}^2. Find the friction force.

Example 4

hard
A 4 kg4\,\text{kg} block on level ground is pushed by a 30 N30\,\text{N} force directed 30∘30^\circ below horizontal. With μk=0.3\mu_k = 0.3 and g=10 m/s2g = 10\,\text{m/s}^2, find the acceleration.

Example 5

medium
A block slides on level ground and decelerates at 3 m/s23\,\text{m/s}^2 due to friction alone. Find μk\mu_k (g=10 m/s2g = 10\,\text{m/s}^2).

Example 6

challenge
A 2 kg block is launched up a 37∘37^\circ incline at 10 m/s10\,\text{m/s}. With μk=0.25\mu_k = 0.25 and g=10 m/s2g = 10\,\text{m/s}^2 (sin⁡37∘=0.6\sin 37^\circ = 0.6, cos⁡37∘=0.8\cos 37^\circ = 0.8), how far up the incline does it travel before stopping?

Example 7

medium
A block decelerates at 4 m/s24\,\text{m/s}^2 on level ground due to friction alone (g=10 m/s2g = 10\,\text{m/s}^2). Find μk\mu_k.

Example 8

challenge
A 3 kg block on level ground is pushed with 24 N24\,\text{N} for 2 s2\,\text{s} then released. With μk=0.2\mu_k = 0.2, g=10 m/s2g = 10\,\text{m/s}^2, find the total distance until it stops.

Example 9

hard
Why does dragging a 10 kg10\,\text{kg} box at constant velocity require force only 30 N30\,\text{N}, less than its weight 100 N100\,\text{N}?

Example 10

medium
A block sliding with μk=0.3\mu_k = 0.3 enters a level patch at 6 m/s6\,\text{m/s} and stops due to friction alone (g=10 m/s2g = 10\,\text{m/s}^2). How far does it travel?

Example 11

easy
A box on a horizontal surface has normal force N=50 NN = 50\,\text{N} and μk=0.4\mu_k = 0.4. Find the kinetic friction force as it slides.

Example 12

hard
A 5 kg5\,\text{kg} block has μk=0.4\mu_k = 0.4, g=10 m/s2g = 10\,\text{m/s}^2, and is pulled by a constant horizontal 30 N30\,\text{N} force from rest. Find its speed after 4 m4\,\text{m}.

Example 13

hard
Two blocks of mass 2 kg2\,\text{kg} and 3 kg3\,\text{kg} are connected by a string on a level table, μk=0.2\mu_k = 0.2 both, g=10 m/s2g = 10\,\text{m/s}^2. A 25 N25\,\text{N} force pulls the 3 kg3\,\text{kg} block. Find the acceleration.

Example 14

easy
Kinetic friction on a sliding puck is fk=6 Nf_k = 6\,\text{N} with normal force N=30 NN = 30\,\text{N}. Find μk\mu_k.

Example 15

challenge
A 2 kg2\,\text{kg} block is launched up a 53∘53^\circ incline at 12 m/s12\,\text{m/s}. With μk=0.25\mu_k = 0.25 and g=10 m/s2g = 10\,\text{m/s}^2 (sin⁡53∘=0.8\sin 53^\circ = 0.8, cos⁡53∘=0.6\cos 53^\circ = 0.6), how far up the incline does it travel before stopping?

Example 16

medium
A 10 kg10\,\text{kg} block on level ground is pushed with 40 N40\,\text{N}, μk=0.3\mu_k = 0.3, g=10 m/s2g = 10\,\text{m/s}^2. Find the net horizontal force.

Example 17

medium
A 3 kg3\,\text{kg} block slides 5 m5\,\text{m} on level ground with μk=0.4\mu_k = 0.4, g=10 m/s2g = 10\,\text{m/s}^2. How much heat is generated?

Example 18

medium
A 4 kg4\,\text{kg} block slides on level ground with μk=0.25\mu_k = 0.25 and no applied force (g=10 m/s2g = 10\,\text{m/s}^2). Find its deceleration.

Example 19

medium
A block slides 4 m across a floor against kinetic friction fk=8 Nf_k = 8\,\text{N}. How much mechanical energy is converted to heat?

Example 20

easy
A 50 N50\,\text{N} horizontal push moves a 10 kg10\,\text{kg} box at constant velocity on a level floor. Find μk\mu_k (g=10 m/s2g = 10\,\text{m/s}^2).
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