4PH1

Forces, Movement & Changing Shape

Forces & Motion · 1 question type

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 14% of your exam marks.

stable
High
Stable14%

F = ma, resultant forces and Hooke's Law calculations are high-frequency multi-mark questions.

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)