Gravity: The weight of an object can be calculated using the equation:

The idea

Weight is the force acting on an object because of gravity, and it is measured in newtons (N). It is not the same as mass, which is the amount of matter in the object and is measured in kilograms (kg). The equation that links them is 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, often rounded to 10 N/kg in calculations. For example, a 5 kg bag has weight 5 × 9.8 = 49 N on Earth, but only about 8 N on the Moon where g is about 1.6 N/kg. The mass stays 5 kg everywhere; only the weight changes. This statement introduces the equation you must be able to use and recall.

What earns the marks

• State that weight is a force measured in newtons (N) and mass is measured in kilograms (kg).
• Recall and write the equation weight = mass × gravitational field strength, using W = m × g.
• Substitute values correctly, keeping mass in kg and gravitational field strength in N/kg.
• Calculate weight by multiplying mass by gravitational field strength, for example 5 kg × 9.8 N/kg = 49 N.
• Explain that gravitational field strength changes with location, so weight changes while mass stays constant.

Common mistakes and how to put them right

• Writing weight in kilograms.

  Put it right: Weight is a force, so the unit is newtons (N); mass is in kilograms (kg).

• Using g = 10 N/kg when the question gives 9.8 N/kg.

  Put it right: Use the value stated in the question.

• Dividing instead of multiplying.

  Put it right: The equation is W = m × g, so multiply mass by gravitational field strength.

Exam tips

• Write the equation, substitute the numbers, then give the unit with your answer.
• Check whether the question asks for weight or mass before calculating.
• If g is not given, use 9.8 N/kg unless the question tells you to use 10 N/kg.