Force Calculator

Don't Confuse Mass and Weight

Your mass is always your mass (say, 70 kg), no matter where you are. Your weight is the force gravity exerts on that mass. On Earth: 70 kg × 9.8 m/s² = 686 N. On the Moon: 70 kg × 1.6 m/s² = 112 N. Same mass, different weight because gravity changed.

Units Matter (A Lot)

Mass must be in kilograms, acceleration in m/s². You can't mix pounds with meters/second² and expect a correct answer. Convert everything to SI units first. Common mistake in physics homework? Forgetting to convert grams to kilograms (divide by 1000).

Net Force is What Matters

F=ma gives you the NET force - the total force after accounting for all forces acting on the object. If you push a box with 100 N but friction pushes back with 30 N, the net force is 70 N. That's the F you use in F=ma.

Using Weight Instead of Mass

"My textbook says the object weighs 150 N, so I put 150 for mass." No - weight IS a force (measured in Newtons). To get mass from weight, divide by gravity: 150 N ÷ 9.8 m/s² = 15.3 kg. That's your mass.

Why it happens: People use "weight" when they mean "mass" in everyday language.
Fix: Weight is in Newtons. Mass is in kilograms. Don't mix them up.

Forgetting About Negative Acceleration

Slowing down is negative acceleration (deceleration). A car braking at -5 m/s² experiences a force in the opposite direction of motion. The math still works - just remember that negative acceleration means the force opposes the current motion.

Real consequence: Wrong direction of force in your free body diagram, wrong answer on the test.
Fix: Deceleration = negative acceleration. Force points opposite to motion.

Ignoring Multiple Forces

F=ma uses NET force. If gravity pulls down with 100 N and air resistance pushes up with 20 N, the net downward force is 80 N. Not 100 N. You have to add up all forces (accounting for direction) before using F=ma.

How to avoid: Draw a free body diagram. List all forces. Add them up (with + and - for direction). That's your net F.

What is Newton's Second Law (F=ma)?

Force equals mass times acceleration. F=ma. If you push a 10 kg object with 50 Newtons of force, it'll accelerate at 5 m/s². Heavier objects need more force to accelerate at the same rate. It's the fundamental relationship between force, mass, and motion.

What is a Newton (N)?

A Newton is the unit of force. 1 Newton is the force needed to accelerate 1 kg at 1 m/s². An apple weighs about 1 Newton (gravity pulling it down at 9.8 m/s²). Your car engine produces thousands of Newtons to accelerate the car.

How do you calculate force from mass and acceleration?

Multiply mass (in kg) by acceleration (in m/s²). Example: A 50 kg person on a bike accelerating at 2 m/s² experiences 100 N of force (50 × 2 = 100). Bigger mass or faster acceleration = more force needed.

What's the difference between mass and weight?

Mass is how much stuff you're made of (kg). Weight is the force gravity exerts on that mass (Newtons). Your mass is the same on Earth and the Moon, but your weight is different because gravity is weaker on the Moon. Weight = mass × gravity (9.8 m/s² on Earth).

Why is Earth's gravity 9.8 m/s²?

That's the rate everything accelerates downward due to Earth's gravity. Drop something, and it speeds up by 9.8 meters per second every second (ignoring air resistance). It's roughly 10 m/s² if you're doing quick mental math. On the Moon, it's 1.6 m/s². On Jupiter, 24.8 m/s².