Describe the relationship between the weight of a body and the…

The idea

Weight and gravitational field strength are directly proportional for a body of fixed mass: W = m × g, where W is weight in newtons, m is mass in kilograms and g is gravitational field strength in newtons per kilogram (N/kg). On Earth g is about 9.8 N/kg, so a 2 kg mass weighs about 19.6 N. Doubling g doubles the weight of the same body, and halving g halves it, because mass stays constant. Gravitational field strength varies between bodies: it is weaker on the Moon, so the same astronaut has the same mass but a smaller weight there. A learner should describe this proportional link and use the equation to calculate weight from mass and g.

What earns the marks

• States that weight equals mass multiplied by gravitational field strength, W = m × g.
• Explains that weight is directly proportional to gravitational field strength when mass is constant.
• Uses the unit N/kg for gravitational field strength and links it to newtons per kilogram of mass.
• Applies the relationship to a comparison, such as the smaller weight of the same mass on the Moon where g is weaker.

Common mistakes and how to put them right

• Treats mass as changing when gravitational field strength changes.

  Put it right by stating that mass stays constant while weight changes.

• Uses grams directly in W = m × g.

  Put it right by converting mass to kilograms first.

• Writes the unit of g as newtons.

  Put it right by giving newtons per kilogram, N/kg.

Exam tips

• Quote the equation W = m × g and define each symbol with its unit.
• Convert mass to kilograms before substituting into the equation.
• Use a comparison of two values of g to show the proportional change in weight.