Weight, Mass & g

• Weight is the gravitational force on an object. It always points vertically down, and it is drawn from the object's centre of gravity (the point where the whole weight appears to act).
• $g$ is the gravitational field strength: the force of gravity per kilogram. On Earth $g = 9.81 \text{ N kg}^{-1}$.
• The unit N kg⁻¹ is the same as m s⁻², so $g$ is also the acceleration of free fall from lesson 2.6. Same number, two meanings: force per kilogram, or acceleration when only gravity acts.

• On the Moon, $g \approx 1.6 \text{ N kg}^{-1}$, about one sixth of Earth's value.
• A buggy on the Moon has the same mass, so starting it moving needs the same force as on Earth ($F = ma$ does not care where you are).
• But its weight is six times smaller, so lifting it is much easier.

Mass stays the same wherever the object goes. Weight changes because $g$ changes.