Describe, and use the concept of, weight as the effect of a gravitational field on a mass

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.

Calculating Weight

1. Identify the mass of the object (in kg).
2. Determine the gravitational acceleration $g$ for the location.
3. 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.