Friction Physics Example 1

Follow the full solution, then compare it with the other examples linked below.

Example 1

medium
A 20 kg20 \text{ kg} box sits on a surface with a coefficient of kinetic friction μk=0.3\mu_k = 0.3. What force is needed to pull it at constant velocity?

Solution

  1. 1
    Calculate the normal force: N=mg=20×9.8=196 NN = mg = 20 \times 9.8 = 196 \text{ N}
  2. 2
    Calculate the friction force: fk=μkN=0.3×196=58.8 Nf_k = \mu_k N = 0.3 \times 196 = 58.8 \text{ N}
  3. 3
    At constant velocity, the applied force equals friction: Fapplied=fk=58.8 NF_{\text{applied}} = f_k = 58.8 \text{ N}

Answer

Fapplied=58.8 NF_{\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.

About Friction

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

Learn more about Friction →

More Friction Examples

Example 2 hard

A [formula] block on a surface ([formula]) is pushed with [formula]. What is the acceleration?

Example 3 medium

The coefficient of static friction between a box and the floor is [formula]. If the box weighs [form

Example 4 medium

A [formula] crate is pulled across a level floor at constant speed by a horizontal [formula] force.

← All Friction Examples Friction Concept