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Energy – Forces Doing Work

Gravitational potential and kinetic energy calculations

  • 3 things to know
  • Paper 6

Overview

Gravitational potential energy from mass, field strength and height, why lifting work equals the gain, kinetic energy from mass and speed, and finding speed or mass from an energy.

What you need to know

Answer each one before you reveal it.

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Numbers

Calculate the gravitational potential energy gained when a crane lifts a 250 kg load through 12 m, taking gravitational field strength as 10 N/kg.

Numbers

Calculate the gravitational potential energy gained by a 60 kg climber who ascends 8 m, taking gravitational field strength as 10 N/kg.

Key term

Explain why the work done in lifting an object steadily is equal to its gain in gravitational potential energy.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Calculate the gravitational potential energy gained when a crane lifts a 250 kg load through 12 m, taking gravitational field strength as 10 N/kg.

    Model answer

    ΔGPE = 250 × 10 × 12 = 30 000 J.

  2. 2

    Calculate the gravitational potential energy gained by a 60 kg climber who ascends 8 m, taking gravitational field strength as 10 N/kg.

    Model answer

    ΔGPE = 60 × 10 × 8 = 4800 J.

  3. 3

    An 8 kg box gains 1200 J of gravitational potential energy when it is raised. Calculate the height it is raised through, taking gravitational field strength as 10 N/kg.

    Model answer

    Height = 1200 ÷ (8 × 10) = 15 m.

  4. 4

    Lifting a load through 3 m requires 720 J of work against gravity. Calculate the mass of the load, taking gravitational field strength as 10 N/kg.

    Model answer

    Mass = 720 ÷ (10 × 3) = 24 kg.

  5. 5

    Explain why the work done in lifting an object steadily is equal to its gain in gravitational potential energy.

    Model answer

    The lifting force equals the weight, so force × height is the same as mass × gravitational field strength × height.

  6. 6

    Calculate the gravitational potential energy lost by a 0.15 kg apple that falls 4 m from a tree, taking gravitational field strength as 10 N/kg.

    Model answer

    ΔGPE = 0.15 × 10 × 4 = 6 J.

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