Test yourself
Question
What is the weight of a 45 kg girl on:
- the Earth, where g = 10 N/kg?
- the Moon, where g = 1.6 N/kg?
- 1. The Earth
W = mg
m = 45 kg
g = 10 N/kg
W = 45 x 10
W = 450 N
The weight of the girl on the Earth is 450 N
- 2. The Moon
W = mg
m = 45 kg
g = 1.6 N/kg
W = 45 x 1.6
W = 72 N
The weight of the girl on the Moon is 72 N.
The mass of the girl is the same on the Earth and the Moon because the amount of matter in the girl is the same in both places.
However, the weight of the girl on the Earth is much greater than the weight of the girl on the Moon because the Earth’s pull of gravity is much greater than the Moon’s.
Question
- What is the mass of a person who weighs 120 N on the Moon (g = 1.6 N/kg)?
- What would the mass and weight of the same person be back on Earth where g = 10 N/kg?
1. The Moon
- m = \(\frac{\text{W}}{\text{g}}\)
- W = 120 N
- g = 1.6 N/kg
- m = 120 ÷ 1.6
- m = 75 kg
- The mass of the person on the Moon is 75 kg
2. The Earth
- The mass of the person back on the Earth will also be 75 kg as mass does not change from place to place.
- W = mg
- m = 75 kg
- g = 10 N/kg
- W = 75 x 10
- W = 750 N
- The weight of the person on the Earth is 750 N.
