Weight is measured using a calibrated spring-balance (a newtonmeter).

The idea

Weight is the gravitational force acting on an object, measured in newtons (N). A spring-balance, also called a newtonmeter, measures this force directly. Inside, a spring stretches when a force pulls it; the extension is proportional to the force, so a scale can be calibrated in newtons. To use one, hang the object from the hook or pull the hook with the object, wait for it to settle, and read the value at eye level. The reading is the weight, not the mass. Mass is measured with a balance in kilograms (kg) and does not change with location, but weight changes with gravitational field strength g, because W = m × g. For example, a 2 kg mass has a weight of about 20 N on Earth (g ≈ 10 N/kg), but only 3.2 N on the Moon (g ≈ 1.6 N/kg). A calibrated spring-balance therefore gives different readings on different planets, while a mass balance gives the same mass everywhere.

What earns the marks

• Weight is a force measured in newtons (N), whereas mass is measured in kilograms (kg).
• A spring-balance (newtonmeter) measures weight directly by the extension of a calibrated spring.
• The scale of a spring-balance is calibrated in newtons, often using known weights or a known gravitational field strength.
• To measure weight, the object is attached to the hook or the hook is pulled, and the reading is taken when the pointer is steady.
• The reading should be taken at eye level to avoid parallax error.
• Weight is calculated using W = m × g, where g is the gravitational field strength in N/kg.
• A spring-balance reading changes if g changes, but mass remains constant.

Common mistakes and how to put them right

• Confusing weight with mass.

  Put it right: State the unit and remember W = m × g.

• Reading the spring-balance while it is still moving.

  Put it right: Wait for the reading to settle.

• Assuming a spring-balance measures mass.

  Put it right: Use a mass balance for mass and a spring-balance for weight.

• Reading the scale at an angle, causing parallax error.

  Put it right: Position your eye level with the pointer.

Exam tips

• When asked how to measure weight, name the spring-balance (newtonmeter) and state that it is calibrated in newtons.
• If a question gives mass and asks for weight, use W = m × g and include the correct unit (N).
• To compare weight on Earth and the Moon, calculate both using the appropriate g values and comment on the difference.
• Describe the method clearly: attach the object, wait for it to settle, read at eye level.