Define weight, recall and use the equation: weight (newton, N) = mass…

The idea

Weight is the force of gravity acting on an object, measured in newtons (N); mass is the amount of matter in the object, measured in kilograms (kg), and does not change with location. Gravitational field strength g is the force per unit mass, in N/kg, and on Earth's surface g ≈ 9.8 N/kg. The equation W = m × g links them: a 5 kg bag has weight 5 kg × 9.8 N/kg = 49 N. On the Moon g is smaller (about 1.6 N/kg), so the same 5 kg bag weighs about 8 N while its mass stays 5 kg. Rearranged, m = W ÷ g and g = W ÷ m. Check units: kg × N/kg gives N.

What earns the marks

• States that weight is the force of gravity on an object and is measured in newtons (N).
• States that mass is measured in kilograms (kg) and is unchanged by location.
• Recalls the equation weight = mass × gravitational field strength.
• Uses W = m × g with consistent units to calculate a weight in newtons.
• Rearranges the equation to find mass or gravitational field strength.
• Explains that gravitational field strength is measured in N/kg and equals force per unit mass.

Common mistakes and how to put them right

• Confuses weight with mass and gives weight in kilograms.

  Put it right: Keep weight in newtons and mass in kilograms.

• Uses g = 9.8 N/kg as a universal constant for every planet.

  Put it right: Use the g value given for that location.

• Forgets to convert grams to kilograms before multiplying.

  Put it right: Divide grams by 1000 first.

• Multiplies mass by g but reports the answer in kg.

  Put it right: Label the result in N.

Exam tips

• Write the equation, substitute values with units, then give the answer with the correct unit.
• Check whether the question gives mass in grams and convert to kilograms before calculating.
• When asked to define weight, mention force of gravity and newtons, not just 'how heavy'.