Weight
The gravitational force on a mass.
Formula
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Example
A 10 kg mass weighs ≈ 98 N
How it works
Weight is the force that gravity exerts on an object with mass. The formula W = mg comes from the fact that near Earth’s surface, gravity gives every kilogram of mass the same downward acceleration, about 9.81 m/s², so a larger mass experiences a larger force. This means weight depends on both how much matter an object has and the strength of the gravitational field it is in.
When to use it
Use this formula when you need the weight force of an object near Earth or on another planet/moon, such as in mechanics, balancing forces, or calculating how much load a structure must support. It is also useful when converting a measured mass into a force in newtons.
Worked example, step by step
A suitcase has a mass of 18 kg. Use W = mg W = 18 × 9.81 W = 176.58 N Rounded to a sensible value, the suitcase weighs about 177 N.
Common mistakes to avoid
- Using kilograms as the final unit instead of newtons
- Confusing mass with weight and treating them as the same thing
- Forgetting that g changes on different planets or at different locations
Did You Know?
Weight is a force and changes with gravity; your mass stays the same on the Moon but you weigh six times less.
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