01Why this matters
A person’s mass stays essentially the same on the Moon, but their weight is smaller because the gravitational field is weaker.
02See it
03Understand it
Mass measures inertia and is measured in kilograms. Weight is the gravitational force on a mass, measured in newtons.
Near Earth’s surface, weight is W = mg. We use g = 9.8 N/kg for these examples. This is also 9.8 m/s² as a free-fall acceleration. The local value varies slightly. Weight acts towards the attracting body; it is not measured in kilograms.
04Guided example
Find the weight of a 5 kg object near Earth’s surface.
- Identify m = 5 kg and g = 9.8 N/kg.
- Multiply: W = 5 × 9.8 = 49.
- The weight is 49 N, directed towards Earth’s centre.
05Try it
06Practise it
07Use it in the real world
A 10 kg object weighs about 98 N on Earth. In a field of 1.6 N/kg it weighs 16 N, while its mass remains 10 kg.
Go a little deeper
Apparent weight is associated with support force. In free fall, a person can feel weightless even though gravity and their gravitational weight remain present.
08Quick check
09Summary
Mass measures inertia and is measured in kilograms. Weight is the gravitational force on a mass, measured in newtons. The weight is 49 N, directed towards Earth’s centre.
Before moving on, explain the worked example in your own words. Use the hint on any question you want to revisit.
10Key idea
W = mg
W: weight in N; m: mass in kg; g: gravitational field strength in N/kg.
m = W/g, for g ≠ 0
Find this in the formula library →