GCSE Combined Science (AQA)

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Kinetic & GPE Conversions

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Forces Work done and energy transfer

Kinetic & GPE Conversions

5:23 Work, Kinetic Energy and Gravitaitonal Potential Energy
Spec 4.1.1.1 4.1.1.2 4.5.2
Previous Kinetic Energy Next Measuring Speed
  • When an object is thrown upwards, kinetic energy (KE) is converted to gravitational potential energy (GPE).
  • As a ball gains height, work is done against gravity, converting KE to GPE, causing the ball to lose speed and gain height.
  • When an object falls, GPE is converted back to KE, increasing speed and decreasing height.
  • Assuming no energy loss to surroundings, the change in GPE equals the change in KE.
  • Resistive forces cause energy loss to heat, leading to terminal speed.
  • Without resistive forces, the change in KE equals the change in GPE (EK = EP).
  • KE is calculated as 1/2 MV�� and GPE as MGH.
  • Near the planet's surface, 1/2 MV�� = MGH, and masses can be canceled out to simplify.
  • This equation is a starting point for solving harder problems.
  • Example: A 0.8 kg icicle falls 6 meters; calculate its speed upon hitting the ground.
  • Key information: mass = 0.8 kg, height = 6 m, gravitational field strength = 9.8 m/s��.
  • Calculate initial GPE: GPE = mass �� gravitational field strength �� height = 47.04 joules.
  • Rearrange KE equation to solve for speed: V�� = 2EK/M.
  • Substitute GPE for KE to find speed: V = ���(2EP/M).
  • Calculate speed: V = ���(2 �� 47.04 J / 0.8 kg) = 10.8443 m/s.
  • Round speed to two significant figures: 11 m/s.

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