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Out Into Space

Gravitational and kinetic energy changes

  • 3 things to know
  • Components 01, 02 and 03

Overview

Recall questions on the near-surface potential energy equation, work done by a force, why mass cancels in a landing speed, path independence, and areas under force–distance graphs.

What you need to know

Answer each one before you reveal it.

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Key term

What equation gives the change in gravitational potential energy near the Earth's surface?

Key term

A 70 kg climber ascends 12 m. What is the gain in gravitational potential energy?

Key term

What relationship links work done, force and displacement?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What equation gives the change in gravitational potential energy near the Earth's surface?

    Model answer

    The change is ΔE = mgΔh, where g is the gravitational field strength.

  2. 2

    Why is mgh valid only close to the Earth's surface?

    Model answer

    It assumes the field strength g is constant, which is true only over heights small compared with the radius of the Earth.

  3. 3

    A 70 kg climber ascends 12 m. What is the gain in gravitational potential energy?

    Model answer

    ΔE = mgh = 70 × 9.81 × 12 = 8.2 × 10³ J.

  4. 4

    What relationship links work done, force and displacement?

    Model answer

    The work done is ΔE = FΔs, where the displacement is measured along the direction of the force.

  5. 5

    Why is no work done when a force acts at right angles to the motion?

    Model answer

    There is no displacement in the direction of the force, so the product FΔs along that direction is zero.

  6. 6

    A ball of mass 0.20 kg is dropped from 1.8 m. What is its speed on landing, ignoring air resistance?

    Model answer

    From mgh = ½mv², v = √(2 × 9.81 × 1.8) = 5.9 m s⁻¹.

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