// physics › Newton's Laws
Explore Newton's three laws of motion and solve F = ma for force, mass or acceleration.
F = m a (2nd law)
A mind behind this: Isaac Newton 1642–1727
The first law (inertia) says a body stays at rest or keeps moving at constant velocity unless a net force acts on it. The second law says the net force equals mass times acceleration, F = ma. The third law says every action has an equal and opposite reaction.
Choose what you want to find. To get force, multiply mass by acceleration. To get mass, divide force by acceleration. To get acceleration, divide force by mass. For example, a 3 kg trolley pushed so it accelerates at 4 m/s² needs a net force of 3 × 4 = 12 N.
Use SI units: mass in kilograms (kg), acceleration in metres per second-squared (m/s²) and force in newtons (N). One newton is the force that gives a 1 kg mass an acceleration of 1 m/s².
Because acceleration is force divided by mass. For a fixed force, a larger mass gives a smaller acceleration, so heavier objects respond more sluggishly to the same push.
The action and reaction in the third law act on two different bodies, not the same one, so they do not cancel. When you push off the ground, the ground pushes back on you, and that reaction force is what moves you.