Newton's second law of motion
One of the cornerstones of physics, Newton's second law explains how forces make objects move. It states that the force acting on an object equals its mass times its acceleration — the heavier something is, or the faster you want to accelerate it, the more force you need. This calculator solves that relationship for whichever quantity you are missing.
How to use the calculator
Enter any two of force, mass, and acceleration, and leave the third blank. The calculator finds the missing value. Force is measured in newtons (N), mass in kilograms (kg), and acceleration in metres per second squared (m/s²).
The formula
F = m × a. One newton is defined as the force needed to accelerate a one-kilogram mass at one metre per second squared. To push a 5 kg box so it accelerates at 2 m/s², you need F = 5 × 2 = 10 N. Rearranged, mass = force ÷ acceleration and acceleration = force ÷ mass.
Mass, weight and gravity
A common point of confusion is the difference between mass and weight. Mass is the amount of matter in an object and never changes. Weight is the force gravity exerts on that mass, W = m × g, where g is about 9.81 m/s² on Earth. So a 70 kg person has a constant mass of 70 kg but a weight of about 686 N — and would weigh far less on the Moon, where gravity is weaker, despite having exactly the same mass.