All bodies of matter near the surface of Earth experience gravitational force due to Earth’s gravitational field. This gravitational pull is commonly referred to as the weight.
The weight (W) of a body is the gravitational force exerted on it by Earth.
- SI unit of weight is newton (N). It is a vector quantity.
- Its direction is towards the centre of the Earth or commonly referred to as vertically downwards.
- Weight is measured using a newton-meter.
- Your weight does change slightly from place to place on Earth – at the north pole, one would weigh about 3 N heavier than one near the equator. This is due to the gravitational field strength being slightly DIFFERENT at different places on Earth. Normally, you neglect this difference in your calculations. (If you want to ask why heavier at the poles, it is due to the Earth being slightly flattened at the poles.)
| Mass | Weight |
| It is amount of substance in a body | It is the pull of gravity on the body |
| Scalar quantity | Vector quantity |
| SI unit is kilogram (kg) | SI unit is newton (N) |
| Constant under ordinary circumstances | Changes and depends on its location |
| Measured using a beam balance or electronic balance | Measured using a spring or newton-meter |
Note:
When a body is placed in a region of free space far away from any massive bodies, it experiences no gravitational pull and thus are considered weightless. When this same body is now placed near the surface of the Earth, the body experiences the pull of gravity. This shows that the weight must come from an external source. Hence, it is an external force.
Example
The gravitational field strength on the surface of the Moon is one-sixth that of Earth’s. For a body of 60 kg, deduce its mass and weight on the Moon?
Show/Hide Answer
Mass of body on Moon = 60 kg. (Mass don’t change)
Earth’s gravitational field strength, $g_{Earth} = 10 \, \text{m s}^{-2}$
$$\begin{aligned}\text{Weight of body on moon} &= m \times g_{Moon} \\ &= (60)(\frac{10}{6}) \\ &= 100 \, \text{N}\end{aligned}$$
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