How it works
Newton's second law says the net force on an object equals its mass times its acceleration. One newton is the force that accelerates 1 kg at 1 m/s².
Weight is a force too: it is mass times the local gravitational acceleration. A 10 kg mass weighs 10 × 9.80665 = 98.07 N on Earth, which is 22.05 pounds-force or 10 kilograms-force.
Use the net force, meaning the sum of all forces acting. Pushing a cart with 100 N against 30 N of friction gives a net force of 70 N.
Formula
F = m × a
Questions people ask
How many newtons is a pound-force?
One pound-force is exactly 4.4482216152605 N, the weight of one pound of mass under standard gravity.
What is the difference between mass and weight?
Mass (kg) is the amount of matter and does not change with location. Weight (N) is the gravitational force on that mass, so it is smaller on the Moon even though the mass is the same.
Does F = ma apply at very high speeds?
It is accurate for everyday speeds. Close to the speed of light, special relativity applies and the simple relation no longer holds.