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Forces & Newton's laws
A force is a push or a pull. Three short rules from Isaac Newton tell you exactly what every force will do.
Tug of war
Left pull8 N
Right pull5 N
Net force: -3.0 N
Acceleration: -0.75 m/s²
F = ma · m = 4 kg
Newton's three laws
- Inertia. A thing keeps doing what it's doing — at rest or moving in a straight line — unless a force changes it.
- F = ma. Net force equals mass times acceleration. Twice the force, twice the acceleration. Double the mass, half the acceleration.
- Action = reaction. Every push has an equal and opposite push back. Jump up, the Earth pushes down on your feet — you push back up on the Earth.
Try the slider
Set both pulls equal — the box doesn't move (Law 1). Add 1 N to one side — the box accelerates by F/m (Law 2). The rope on each side pulls equal and opposite on the box (Law 3).
Common forces
- Weight — gravity pulling mass downward (W = mg).
- Normal — the ground pushing back up.
- Friction — sideways force resisting motion.
- Tension — pull along a rope or string.
- Applied — your push.
Net force is what matters
Forces add like vectors. Two equal pulls in opposite directions cancel. What's left over — the net force — is what produces acceleration.