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Force is what changes an object's motion, and Newton's second law ties it to just two things: how much mass you're pushing and how fast you're accelerating it — F = m × a. Double the mass or double the acceleration and you double the force.
Choose what you want to find, enter the two values you know, and get the answer in newtons, kilograms or m/s².
How is it calculated?
Newton’s second law, three ways
| Solve for | Formula |
|---|---|
| Force F (N) | mass × acceleration |
| Mass m (kg) | force ÷ acceleration |
| Acceleration a (m/s²) | force ÷ mass |
The unit of force is the newton (N): one newton accelerates one kilogram at one metre per second squared.
Weight is a force
The weight of an object is the gravitational force on it: W = m × g, where g ≈ 9.81 m/s² on Earth. A 10 kg mass weighs about 98 N. That's why bathroom scales, which really measure force, are calibrated to show mass — they divide by g for you.
Mass is not weight
Mass (kg) is how much matter an object contains and never changes; weight (N) is the force gravity exerts on it and depends on where you are. On the Moon your mass is identical but your weight is about one sixth, because g is smaller. Keep the two straight when reading a problem.
Where it helps
Physics homework, working out the thrust needed to accelerate a vehicle, or the weight of a load. For the energy of a moving object rather than the force, use a kinetic energy calculator.
Worked example
A 1,200 kg car accelerates at 3 m/s². The driving force needed is F = m × a = 1,200 × 3 = 3,600 N. Turn it around: if that same car's engine can deliver 6,000 N, its acceleration is a = F ÷ m = 6,000 ÷ 1,200 = 5 m/s². And the car's weight (the force gravity pulls it down with) is 1,200 × 9.81 ≈ 11,772 N.
FAQ
What is Newton’s second law?+
It states that force equals mass times acceleration: F = m × a. The net force on an object determines how quickly its velocity changes.
What is a newton?+
The newton (N) is the SI unit of force: the force that accelerates a 1 kg mass at 1 m/s². The weight of a small apple is roughly 1 N.
How do I calculate the weight of an object?+
Weight is a force: multiply the mass by gravity, W = m × g, with g ≈ 9.81 m/s² on Earth. A 10 kg mass weighs about 98 N.
What is the difference between mass and weight?+
Mass (kg) is the amount of matter and is constant; weight (N) is the gravitational force on that mass and changes with location. On the Moon your weight is about one sixth of Earth’s.
How do I find acceleration from force?+
Divide the force by the mass: a = F ÷ m. A 3,600 N force on a 1,200 kg car gives 3 m/s². Choose "Acceleration" mode and enter the force and mass.