A 20kg box has μ_k = 0.3. What applied force is needed to accelerate it at 2 m/s²?

F_net = ma = 20×2=40N; F_applied = F_net + F_friction = 40 + 0.3×20×9.8.

What is the minimum force to keep a 20kg box moving at constant velocity with μ_k = 0.3?

Constant velocity → F_net = 0 → F_applied = F_friction = μmg.

Why does a heavier box accelerate the same as a lighter one at the same force/mass ratio?

F/m = a; if you scale both F and m by the same factor, a is unchanged.

Free~15 min

Forces and Motion: Basics

Push objects and observe Newton's laws in action

How it works

Newton's Second Law states that the net force on an object equals its mass times acceleration. Friction opposes motion and depends on the normal force and friction coefficient. Static friction prevents motion up to a maximum threshold; kinetic friction acts during sliding. The force diagram (free body diagram) shows all forces acting on the object, and their vector sum gives the net force.

Step-by-step

Use the force arrows to push the object.
The free body diagram updates in real time.
Enable the force chart to see applied force, friction force, and net force over time.
Change surfaces to see different friction coefficients in action.

Key formulas

$F_{net} = ma$ Newton's Second Law
$F_{friction} = \mu_k mg$ Kinetic friction force
$F_{net} = F_{applied} - F_{friction}$ Net force

Frequently asked questions

A 20kg box has μ_k = 0.3. What applied force is needed to accelerate it at 2 m/s²?: F_net = ma = 20×2=40N; F_applied = F_net + F_friction = 40 + 0.3×20×9.8.
What is the minimum force to keep a 20kg box moving at constant velocity with μ_k = 0.3?: Constant velocity → F_net = 0 → F_applied = F_friction = μmg.
Why does a heavier box accelerate the same as a lighter one at the same force/mass ratio?: F/m = a; if you scale both F and m by the same factor, a is unchanged.