Define weight, describe how it is measured and describe the relationship…

The idea

Weight is the force of gravity acting on an object, measured in newtons. You measure it with a newtonmeter, hanging the object from the hook so the spring stretches and the scale reads the force. Weight depends on mass and on the gravitational field strength g, the force per unit mass, through W = m × g. At the Earth's surface g is about 10 N/kg, so a 2 kg mass weighs 2 × 10 = 20 N. On the Moon g is smaller, so the same mass has a smaller weight while its mass stays the same. Mass is measured in kilograms with a balance; weight is a force measured in newtons.

What earns the marks

• Defines weight as the force of gravity acting on an object, measured in newtons.
• States that weight is measured with a newtonmeter.
• States the relationship W = m × g, where g is the gravitational field strength.
• Uses g = 10 N/kg at the Earth's surface to calculate a weight from a mass.
• Distinguishes mass, measured in kilograms and unchanged by location, from weight, which changes with g.

Common mistakes and how to put them right

• Describes weight as measured in kilograms.

  Put it right: State instead that weight is a force measured in newtons and mass is measured in kilograms.

• Uses a balance to measure weight.

  Put it right: Use a newtonmeter instead, since a balance measures mass.

• Treats mass as changing on the Moon.

  Put it right: Keep the mass the same and change only the weight because g is smaller.

Exam tips

• Write the equation as W = m × g before substituting values.
• Check that the mass is in kilograms before multiplying by 10 N/kg.
• Give the unit of newtons with every weight answer.