Kinetic Friction Examples in Physics

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Kinetic Friction.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Physics.

Concept Recap

The friction force that opposes sliding motion between two surfaces in contact; in the simple dry-friction model its magnitude is $f_k = \mu_k N$ , directed opposite to the motion.

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

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How to Use These Examples

Read the first worked example with the solution open so the structure is clear.
Try the practice problems before revealing each solution.
Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Kinetic Friction asks students to choose the object, list external interactions, and reason from the resulting force or torque pattern.

Common stuck point: Students often know a formula related to kinetic friction but skip the recognition step: Have I isolated one system and listed the external forces or torques acting on it before applying a law? That leads to a correct-looking substitution attached to the wrong physical model.

Sense of Study hint: Ask: Have I isolated one system and listed the external forces or torques acting on it before applying a law?

Worked Examples

Example 1

medium

6\,\text{kg}

block is pushed with

30\,\text{N}

horizontally on level ground with

\mu_k = 0.2

and

g = 10\,\text{m/s}^2

. Find its acceleration.

Answer

a = 3\ \text{m/s}^2

First step

f_k = \mu_k m g = 0.2 \cdot 6 \cdot 10 = 12\,\text{N}

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Example 2

medium

4\,\text{kg}

crate is pushed by a horizontal

30\,\text{N}

force and accelerates at

3\,\text{m/s}^2

on level ground (

g = 10\,\text{m/s}^2

). Find

\mu_k

Example 3

hard

4\,\text{kg}

block on level ground is pushed by a

30\,\text{N}

force directed

30^\circ

below horizontal. With

\mu_k = 0.3

and

g = 10\,\text{m/s}^2

, find the acceleration.

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy

A 10 kg crate slides on a level floor with coefficient of kinetic friction

\mu_k = 0.3

. Take

g = 9.8\,\text{m/s}^2

. Find the kinetic friction force.

Find the kinetic friction force on the sliding crate.

Example 2

easy

A box on a horizontal surface has normal force

N = 50\,\text{N}

and

\mu_k = 0.4

. Find the kinetic friction force as it slides.

Find the kinetic friction force given N = 50 N and μ_k = 0.4.

Example 3

easy

A 4 kg book slides on a table,

\mu_k = 0.25

g = 9.8\,\text{m/s}^2

. Find kinetic friction.

Find the kinetic friction force on the sliding book.

Example 4

easy

Kinetic friction on a sliding puck is

f_k = 6\,\text{N}

with normal force

N = 30\,\text{N}

. Find

\mu_k

Given f_k = 6 N and N = 30 N, find μ_k = f_k / N.

Example 5

easy

A 2 kg block slides on level ground,

\mu_k = 0.5

g = 10\,\text{m/s}^2

. Find the friction force.

Find the kinetic friction force on the 2 kg sliding block.

Example 6

easy

Which coefficient applies to an object that is currently sliding:

\mu_s

\mu_k

Example 7

easy

A sled slides with

N = 100\,\text{N}

and

\mu_k = 0.15

. Find kinetic friction.

Find the kinetic friction force on the sliding sled.

Example 8

easy

Does the kinetic friction force on a block change if it speeds up from 2 m/s to 4 m/s (same surface)?

Example 9

medium

A 5 kg block slides on level ground,

\mu_k = 0.2

g = 9.8\,\text{m/s}^2

, with no applied force. Find its deceleration.

Only friction acts; find the deceleration a = f_k / m.

Example 10

medium

A 3 kg block is pushed with

20\,\text{N}

horizontally on level ground,

\mu_k = 0.3

g = 10\,\text{m/s}^2

. Find its acceleration.

Net force = 20 N − 9 N = 11 N; find acceleration.

Example 11

medium

A block slides 4 m across a floor against kinetic friction

f_k = 8\,\text{N}

. How much mechanical energy is converted to heat?

Example 12

medium

A 6 kg block slides down a

30^\circ

incline,

\mu_k = 0.2

g = 10\,\text{m/s}^2

. Find the kinetic friction force.

Example 13

medium

A 2 kg puck moving at

6\,\text{m/s}

slides to rest over

9\,\text{m}

on level ice. Find

\mu_k

(

g = 10\,\text{m/s}^2

Example 14

medium

A 10 kg crate is pulled at constant velocity on level ground by a horizontal rope. If

\mu_k = 0.25

and

g = 9.8\,\text{m/s}^2

, find the rope tension.

Constant velocity: T = f_k; find the rope tension.

Example 15

medium

A block slides on level ground and decelerates at

3\,\text{m/s}^2

due to friction alone. Find

\mu_k

(

g = 10\,\text{m/s}^2

Example 16

medium

A 4 kg block is pushed at

30\,\text{N}

on level ground,

\mu_k = 0.4

g = 10\,\text{m/s}^2

. Will it accelerate, and what is the net horizontal force?

Find the net horizontal force: F_net = 30 N − 16 N.

Example 17

medium

A hockey puck slides

20\,\text{m}

and loses

40\,\text{J}

of kinetic energy to friction. Find the kinetic friction force.

Example 18

challenge

A 2 kg block is launched up a

37^\circ

incline at

10\,\text{m/s}

. With

\mu_k = 0.25

and

g = 10\,\text{m/s}^2

(

\sin 37^\circ = 0.6

\cos 37^\circ = 0.8

), how far up the incline does it travel before stopping?

Going up: both W sin37° and f_k act down the slope. Find stopping distance.

Example 19

challenge

A 5 kg block slides on level ground at

8\,\text{m/s}

. After

4\,\text{m}

it reaches a rough patch and stops in another

2\,\text{m}

. On the rough patch find

\mu_k

, given the first

4\,\text{m}

is frictionless (

g = 10\,\text{m/s}^2

On the rough patch only: friction decelerates the block from 8 m/s to rest in 2 m.

Example 20

challenge

A 3 kg block on level ground is pushed with

24\,\text{N}

for

2\,\text{s}

then released. With

\mu_k = 0.2

g = 10\,\text{m/s}^2

, find the total distance until it stops.

Phase 1 (pushed): F_net = 24 N − 6 N = 18 N, a = 6 m/s².

Example 21

easy

20\,\text{kg}

box slides on a level floor with

\mu_k = 0.2

and

g = 10\,\text{m/s}^2

. Find the kinetic friction force.

Example 22

easy

A block on level ground has

N = 80\,\text{N}

and

\mu_k = 0.35

. Find the kinetic friction force.

Example 23

easy

5\,\text{kg}

crate sliding on a level surface experiences

f_k = 12\,\text{N}

(

g=10\,\text{m/s}^2

). Find

\mu_k

Example 24

easy

1\,\text{kg}

puck slides on level ice with

\mu_k = 0.05

and

g = 9.8\,\text{m/s}^2

. Find the kinetic friction force.

Example 25

easy

50\,\text{N}

horizontal push moves a

10\,\text{kg}

box at constant velocity on a level floor. Find

\mu_k

(

g = 10\,\text{m/s}^2

Example 26

medium

4\,\text{kg}

block slides on level ground with

\mu_k = 0.25

and no applied force (

g = 10\,\text{m/s}^2

). Find its deceleration.

Example 27

medium

A block sliding with

\mu_k = 0.3

enters a level patch at

6\,\text{m/s}

and stops due to friction alone (

g = 10\,\text{m/s}^2

). How far does it travel?

Example 28

medium

3\,\text{kg}

block slides

5\,\text{m}

on level ground with

\mu_k = 0.4

g = 10\,\text{m/s}^2

. How much heat is generated?

Example 29

medium

2\,\text{kg}

block slides down a

30^\circ

incline with

\mu_k = 0.1

and

g = 10\,\text{m/s}^2

. Find the kinetic friction force.

Example 30

medium

5\,\text{kg}

box is pulled at constant velocity on level ground by a horizontal rope,

\mu_k = 0.3

g = 10\,\text{m/s}^2

. Find the rope tension.

Example 31

medium

10\,\text{kg}

block on level ground is pushed with

40\,\text{N}

\mu_k = 0.3

g = 10\,\text{m/s}^2

. Find the net horizontal force.

Example 32

medium

A puck on level ice slides

25\,\text{m}

and loses

20\,\text{J}

of kinetic energy to friction. Find the kinetic friction force.

Example 33

medium

5\,\text{kg}

block slides at

4\,\text{m/s}

on level ground with

\mu_k = 0.2

g = 10\,\text{m/s}^2

. How far does it travel before stopping?

Example 34

medium

A box is pulled across level ground by a rope at

20^\circ

above horizontal with tension

50\,\text{N}

. The block has mass

8\,\text{kg}

\mu_k = 0.3

g = 10\,\text{m/s}^2

. Find the normal force.

Example 35

hard

2\,\text{kg}

block slides down a

37^\circ

incline with

\mu_k = 0.25

and

g = 10\,\text{m/s}^2

(

\sin 37^\circ = 0.6

\cos 37^\circ = 0.8

). Find the block's acceleration down the slope.

Example 36

hard

3\,\text{kg}

block slides up a

30^\circ

incline at

8\,\text{m/s}

with

\mu_k = 0.2

and

g = 10\,\text{m/s}^2

. How far up does it travel before momentarily stopping?

Example 37

hard

A block initially at

10\,\text{m/s}

slides on level ground, decelerating at

2.5\,\text{m/s}^2

due to friction. After

2\,\text{s}

, find the speed and the distance traveled.

Example 38

hard

5\,\text{kg}

block has

\mu_k = 0.4

g = 10\,\text{m/s}^2

, and is pulled by a constant horizontal

30\,\text{N}

force from rest. Find its speed after

4\,\text{m}

Example 39

hard

Two blocks of mass

2\,\text{kg}

and

3\,\text{kg}

are connected by a string on a level table,

\mu_k = 0.2

both,

g = 10\,\text{m/s}^2

. A

25\,\text{N}

force pulls the

3\,\text{kg}

block. Find the acceleration.

Example 40

challenge

2\,\text{kg}

block is launched up a

53^\circ

incline at

12\,\text{m/s}

. With

\mu_k = 0.25

and

g = 10\,\text{m/s}^2

(

\sin 53^\circ = 0.8

\cos 53^\circ = 0.6

), how far up the incline does it travel before stopping?

Example 41

challenge

3\,\text{kg}

block slides on level ground at

9\,\text{m/s}

and crosses a

5\,\text{m}

rough patch with

\mu_k = 0.4

(

g = 10\,\text{m/s}^2

) before continuing on frictionless ground. Find the speed after the rough patch.

Example 42

challenge

4\,\text{kg}

block is pushed horizontally with

40\,\text{N}

for

3\,\text{s}

on level ground (

\mu_k = 0.25

g = 10\,\text{m/s}^2

), then released. Find the total distance traveled until it stops.

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Related Concepts

Friction Normal Force Net Force Work

Background Knowledge

These ideas may be useful before you work through the harder examples.

frictionnormal force