Practice

W = mg Formula – How to Calculate Weight (Practice)

W = mg — Weight as gravitational force near Earth

Weight is the gravitational force on a mass near a planet: W = m g. Mass (kg) stays the same on the Moon; weight (N) changes with g.

Guide — how to use this formula

1. 1. Identify mass in kilograms (convert grams if needed).
2. 2. Use the stated g.
3. 3. Compute W = m g in newtons.
4. 4. For PE problems, PE = W h = mgh.

Derivation

Near Earth’s surface, gravity is nearly uniform with field strength g:

1. Newton’s second law with gravitational acceleration g:
   F = ma = mg
2. That force is the weight:
   W = mg

Use the g given in the problem (9.8, 9.81, or 10). Weight is never reported in kilograms.

Examples

Example: 5 kg on Earth

g = 9.8 → W = 49 N.

Example: same mass on Moon

m = 60 kg, g = 1.6 → W = 96 N (not 600 N).

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