Solve F = ma for force, mass or acceleration in newtons, kilograms and m/s², or switch modes to find an object’s weight on Earth, the Moon or Mars, and the friction force from a coefficient of friction.
Force formulas
Worked example
A 1,200 kg car accelerates at 3 m/s². The net force is F = 1200 × 3 = 3,600 N. If the engine delivers 5,000 N, the remaining 1,400 N is overcoming friction and air resistance. A 70 kg person weighs 70 × 9.81 = 687 N on Earth but only 113 N on the Moon: mass stays the same, weight changes.
Newton’s three laws
- An object stays at rest or moves at constant velocity unless a net force acts on it.
- Net force equals mass times acceleration (F = ma).
- Every action has an equal and opposite reaction.
Frequently asked questions
How do you calculate force?
Multiply mass by acceleration: F = ma. A 2 kg object accelerating at 5 m/s² needs a net force of 10 N.
What is the difference between mass and weight?
Mass (kg) is the amount of matter and never changes. Weight is the force of gravity on that mass (W = mg, in newtons) and depends on where you are.
What is a newton?
The SI unit of force: the force that accelerates 1 kg at 1 m/s². About the weight of a small apple.
How do I calculate friction?
Multiply the coefficient of friction μ by the normal force N. On a flat surface, N equals the object’s weight mg.
Related tools
Free educational tool by HawkInc. Results are calculated in your browser and rounded for display; check critical work by hand or with a second method.