PhysicsExam code: 4PH1
Forces, Movement & Changing Shape
Forces & Motion
Definition
- Weight is the pull that a gravitational field exerts on any object that has mass
- Because weight is a force, it carries a direction (always towards the centre of the planet), making weight a ; weight is measured in newtons (N)
- Mass is the amount of matter in an object, a measured in kilograms (kg)
- Mass and weight are not the same physical quantity even though everyday language often confuses them
The equation
W = m × g
- where:
- W = weight (N)
- m = mass (kg)
- g = (N/kg)
- g on Earth is 10 N/kg (also equal to the acceleration of free fall, 10 m/s², because these two numerical values match when they describe the same physical effect from two different angles)
- g varies between planets and moons: a stronger gravitational field gives a larger weight for the same mass
- Mass and weight are directly proportional to one another in a given gravitational field, so doubling the mass doubles the weight
Worked example
Calculating weight and the unit mark
A satellite component has a mass of 35 kg. Calculate its weight on Earth, where g = 10 N/kg. Give the unit.
Solution:
- Identify the equation: W = m × g
- Substitute: W = 35 × 10
- Evaluate: W = 350
- State the unit: N (newtons)
When a question says "give the unit", the unit is marked on its own, so 350 with no unit loses that mark even though the number is right. Check that the mass is in kilograms before you multiply: a mass given in grams has to be divided by 1000 first, and a power-of-ten slip costs a mark.