2.8 - Weight

Weight and Gravitational Field Strength

• Weight, $W$, is the gravitational force acting on an object. It is a vector directed towards Earth’s centre.
• When a released object accelerates under gravity, Newton’s Second Law gives $F=ma$, with $F=W$ and $a=g$:

• Weight is measured in N; mass, $m$, is measured in kg.
• Gravitational field strength, $g$, is gravitational force per unit mass:

• Its unit is $\mathrm{N}\mathrm{k}{\mathrm{g}}^{-1}$. At Earth’s surface, $g=9.81\mathrm{N}\mathrm{k}{\mathrm{g}}^{-1}$: each 1 kg mass experiences a gravitational force of 9.81 N.

Weight on Earth and the Moon

• The Moon’s gravitational field strength is one-sixth of Earth’s. A spacesuit therefore weighs less on the Moon, making it easier for the astronaut to move.
• For a spacesuit of mass 81.65 kg: