All courses › Foundations of Physics › Newton's laws

Newton's laws

Newton's three laws link force and motion. The first law says a body with no net force keeps a constant velocity. The second law relates the net force to the acceleration. The third law says forces always come in pairs acting on two different bodies, so they never cancel each other.

\sum F = ma

Newton's second law

F_{A\to B} = -F_{B\to A}

Newton's third law

Symbols

$F$	force	N
$m$	mass	kg
$a$	acceleration	m/s²

Example

A block with $m = 5.0$ kg is acted on by a net force of $20$ N. The acceleration is $a = F/m = 20/5.0 = 4.0$ m/s².

Action and reaction (3rd law) act on two different bodies, so they can never appear in the same free-body diagram and cancel.

Practise forces and Newton's laws for free →

← Projectile motion · Weight and normal force →

Part of Foundations of Physics: Forces and Newton's laws.