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Mass and weight

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Lesson notes

Mass and Weight – Key Definitions

  • **Mass** is the amount of matter in an object, measured in **kilograms (kg)**.
  • **Weight** is a **downwards force** due to gravity, measured in **newtons (N)**.
  • Mass stays the **same** everywhere; weight **changes** with gravitational field strength.
  • Gravitational field strength (**g**) is the force per unit mass, measured in **N/kg**.
  • On Earth, g = **9.8 N/kg**; on the Moon, g = **1.6 N/kg**.

The Weight Equation

  • Weight = Mass × Gravitational field strength (W = m × g).
  • To calculate weight: multiply mass (kg) by g (N/kg).
  • To calculate mass: rearrange to m = W ÷ g.
  • To calculate g: rearrange to g = W ÷ m.
  • Always include **units** in your answer (N for weight, kg for mass).

Worked Example – Calculating Weight

  • Example: A 20 kg object on the Moon (g = 1.6 N/kg).
  • Write equation: Weight = Mass × g.
  • Substitute: 20 kg × 1.6 N/kg = **32 N**.
  • The weight is 32 N.

Worked Example – Calculating Mass

  • Example: An object weighs 0.74 N on Mars (g = 3.7 N/kg).
  • Rearrange: Mass = Weight ÷ g.
  • Substitute: 0.74 N ÷ 3.7 N/kg = **0.2 kg**.
  • The mass is 0.2 kg.

Key Points to Remember

  • Weight is a **force**; it always acts **downwards**.
  • Gravitational field strength depends on the **mass of the planet**.
  • Even you attract other objects, but your mass is too small for a noticeable force.
  • On Earth, a 1 kg mass has a weight of **9.8 N**.

Particle arrangement in solids, liquids and gases – mass is the amount of matter, weight depends on gravity.

Particle arrangementSolidLiquidGas

Slides

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Practice questions

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  1. 1.What is the equation linking weight (W), mass (m), and gravitational field strength (g)?

    Easy
    • AW = m × g
    • BW = m ÷ g
    • CW = g ÷ m
    • DW = m + g
  2. 2.Which of the following statements is true?

    Easy
    • AThe mass of an object is the same everywhere.
    • BThe weight of an object is the same everywhere.
    • CMass changes with location, but weight does not.
    • DWeight is measured in kilograms.
  3. 3.What is the weight of a 0.5 kg book on Earth where g = 9.8 N/kg?

    Easy
    • A4.9 N
    • B0.5 N
    • C9.8 N
    • D5.0 N
  4. 4.A Lunar Rover has a mass of 210 kg. What is its weight on the Moon where g = 1.6 N/kg?

    Easy
    • A336 N
    • B131 N
    • C210 N
    • D1.6 N
  5. 5.A 2 kg rock on Venus has a weight of 17.6 N. What is the gravitational field strength on Venus?

    Medium
    • A8.8 N/kg
    • B35.2 N/kg
    • C0.11 N/kg
    • D2.0 N/kg
  6. 6.On Pluto, g = 0.5 N/kg. What is the weight of a 4 kg object?

    Medium
    • A2.0 N
    • B0.5 N
    • C8.0 N
    • D4.5 N
  7. 7.An object on Ceres has weight 0.81 N and mass 3 kg. What is g on Ceres?

    Medium
    • A0.27 N/kg
    • B2.43 N/kg
    • C3.7 N/kg
    • D0.81 N/kg
  8. 8.Dr Edmunds has mass 70 kg and weight 259 N on Mercury. What is g on Mercury?

    Medium
    • A3.7 N/kg
    • B0.27 N/kg
    • C9.8 N/kg
    • D259 N/kg

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