What Is Force? Newtons Explained for Beginners

Force is what causes an object to accelerate, and understanding it clears up a mix-up that also comes up in Weight vs Mass: why a unit like “kilogram-force” exists at all, when kilograms already measure mass.

What Is Force?

Force is described by Newton’s second law: force equals mass times acceleration. It’s what changes an object’s motion, whether that’s speeding it up, slowing it down, or changing its direction.

Common Force Units

• Newton (N): the SI unit of force, the amount needed to accelerate a 1 kg mass at 1 meter per second squared.
• Kilonewton (kN): 1,000 newtons, used for larger forces like structural loads.
• Pound-force (lbf): the US customary unit of force.
• Kilogram-force (kgf): the force exerted by standard gravity on a 1 kg mass.

Why “kgf” Is a Unit of Force, Not Mass

A kilogram-force isn’t a mass measurement despite the name; it’s the specific amount of force that Earth’s standard gravity exerts on a 1 kg mass. That’s exactly the mass-versus-weight distinction covered in Weight vs Mass: mass stays the same everywhere, but the force gravity exerts on it, its weight, does not.

Worked Example

Common Mistakes

• Treating kgf and kg as interchangeable. kg measures mass; kgf measures the force gravity exerts on that mass. They’re related but not the same kind of quantity.
• Assuming force units convert like mass units. Because kgf and lbf both depend on standard gravity in their definitions, they aren’t simply scaled versions of kg and lb the way, say, kg and g are.
• Ignoring direction. Force has both size and direction; a unit conversion only handles the size, not which way the force is acting.

Use the Force Converter

The Force Converter converts between newtons, kN, lbf, kgf, and more instantly.