Newton's Second Law — Physics.explained

§ 01

The equation

EQ.NEWTONS-SECOND-LAW

F = ma

§ 02

What it solves

Relates net force, mass, and acceleration. Given any two of the three quantities, the equation yields the third. It is the foundation of classical mechanics force analysis.

§ 03

When to use it

Any low-speed (non-relativistic) problem where you draw a free-body diagram and sum all forces on a single object. Applies to both translational and (in adapted form) rotational dynamics.

§ 04

When NOT to use it

Does not apply at speeds approaching c — use relativistic mechanics. Also breaks down for objects of variable mass (rocket equation requires the full momentum form dp/dt = F). Does not directly account for internal stresses; use it only on the system boundary.

§ 05

Common mistakes

Using weight (mg) where mass (m) is required, or vice versa. Applying F = ma to the whole system instead of isolating a single object. Forgetting that F is the net force — omitting friction, tension, or normal force components.

§ 06

Topics that use this equation

newtons three laws