Weight, Mass & g

• Weight is the gravitational force on an object. It points vertically downward near Earth's surface, 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.

Earth: larger g larger weightMoon: smaller g smaller weight

• 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. Therefore, the same resultant force gives it the same acceleration as on Earth. Surface friction may still be different.
• 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.