Friction Examples in Physics

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

A contact force that opposes the relative motion or tendency of motion between two surfaces in contact.

The resistance you feel when sliding something across a rough surface — it always acts opposite to motion.

Read the full concept explanation →

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: Friction always acts opposite to the direction of motion or intended motion.

Common stuck point: Static friction (before moving) is usually stronger than kinetic friction (while moving).

Worked Examples

Example 1

medium
A 20 \text{ kg} box sits on a surface with a coefficient of kinetic friction \mu_k = 0.3. What force is needed to pull it at constant velocity?

Solution

  1. 1
    Calculate the normal force: N = mg = 20 \times 9.8 = 196 \text{ N}
  2. 2
    Calculate the friction force: f_k = \mu_k N = 0.3 \times 196 = 58.8 \text{ N}
  3. 3
    At constant velocity, the applied force equals friction: F_{\text{applied}} = f_k = 58.8 \text{ N}

Answer

F_{\text{applied}} = 58.8 \text{ N}
Kinetic friction opposes the motion of sliding objects. At constant velocity, the net force is zero, so the applied force must exactly balance friction.

Example 2

hard
A 15 \text{ kg} block on a surface (\mu_k = 0.25) is pushed with 80 \text{ N}. What is the acceleration?

Practice Problems

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

Example 1

medium
The coefficient of static friction between a box and the floor is \mu_s = 0.4. If the box weighs 98 \text{ N}, what is the maximum static friction force?

Example 2

medium
A 12 \text{ kg} crate is pulled across a level floor at constant speed by a horizontal 29.4 \text{ N} force. What is the coefficient of kinetic friction?
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Background Knowledge

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

forcenormal force