1.2 Motion – Weight
What is Weight?
Weight is the force exerted on an object by the gravitational field of a planet or moon. It is not the same as mass, which is a measure of how much matter an object contains.
Think of weight as the “push” you feel when you stand on a scale. The scale measures how hard the Earth pulls you down.
⚖️ Key Formula: \$W = m\,g\$
Weight vs Mass
- Mass (m) – constant for an object, measured in kilograms (kg).
- Weight (W) – variable depending on the gravitational field, measured in newtons (N).
- On Earth: \$g \approx 9.81\,\text{m/s}^2\$.
- On the Moon: \$g \approx 1.62\,\text{m/s}^2\$ – an object weighs only about 1/6 of its Earth weight.
Weight in Different Gravitational Fields
Because \$W = m\,g\$, the same mass will have different weights on different celestial bodies.
| Location | g (m/s²) | Weight of 10 kg (N) |
|---|
| Earth | 9.81 | \$98.1\$ |
| Moon | 1.62 | \$16.2\$ |
| Mars | 3.71 | \$37.1\$ |
Calculating Weight
- Identify the mass of the object (in kg).
- Determine the gravitational acceleration \$g\$ for the location.
- Multiply: \$W = m\,g\$.
Example: A 5 kg backpack on Earth weighs \$5 \times 9.81 = 49.05\$ N.
Exam Tips for IGCSE Physics 0625
- Always state the units – kg for mass, N for weight.
- Remember \$g\$ is approximately \$9.81\,\text{m/s}^2\$ on Earth unless otherwise specified.
- Use the symbol \$W\$ for weight and \$m\$ for mass in equations.
- When given a weight, you can find mass by rearranging: \$m = W/g\$.
- Check your answers for reasonableness – a 70 kg person should weigh around 700 N on Earth.