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IGCSEPhysicsEdexcelPhysics (Modular)Unit 1Revision Notes3. Energy Resources & Energy TransfersWork, Power & Energy ResourcesWork, GPE & KE

Work, GPE & KE (Edexcel IGCSE Physics (Modular))

Revision Note

Leander Oates

Written by: Leander Oates

Reviewed by: Caroline Carroll

Work, GPE & KE

  • GPE is gravitational potential energy

  • KE is kinetic energy

  • In many situations, energy is transferred between the GPE and KE stores

  • Whenever mechanical work is done (when a force acts over a distance), energy is transferred mechanically 

    • This is a consequence of conservation of energy

  • The amount of energy transferred (in joules) is equal to the work done (in joules or newton-metres)

energy transferred (J) = work done (J or N m)

GPE and KE calculations

  • In a perfect energy transfer, there is no wasted energy

  • Energy transfers can be assumed to be perfect if the wasted energy transfer is negligible

    • Some exam questions will state to ignore air resistance for example

    • In reality, there is no such thing as a perfect energy transfer

  • Ignoring wasted energy transfers is helpful in calculations because it allows energy values to be equated

  • Pendulums are often used as examples of perfect energy transfers

    • All of the energy in the kinetic store of the pendulum is transferred mechanically into its gravitational potential store

    • And then all of the energy in the gravitational potential store of the pendulum is transferred mechanically into its kinetic store

    • Energy is transferred back and forth between these two stores as the pendulum swings

    • Therefore, it can be said that:

K E subscript t o t a l end subscript space equals space G P E subscript t o t a l end subscript

Worked Example

The diagram shows a rollercoaster going down a track.

The rollercoaster takes the path A → B → C → D.

WE - Energy transfers question image, downloadable AS & A Level Physics revision notes

The rollercoaster begins at a height of 15 m above the ground and ends at ground level.

Breaking to stop the ride begins after it passes position D.

The mass of the rollercoaster is 100 kg.

Calculate the maximum speed of the rollercoaster at position D. Ignore any frictional effects before passing point D.

Answer: 

Step 1: List the known quantities

  • Height, h = 15 m

  • Mass, m = 100 kg

  • Gravitational field strength, g = 10 N/kg

Step 2: Write out the equation for gravitational potential energy

increment G P E space equals space m space cross times space g space cross times space increment h

Step 3: Calculate the gravitational potential energy

increment G P E space equals space 100 space cross times space 10 space cross times space 15

increment G P E space equals space 15 space 000 space straight J

Step 4: Use energy equivalency to equate the gravitational potential and kinetic energy

  • Frictional effects are to be ignored; therefore, a perfect energy transfer can be assumed

increment G P E space equals space K E

Step 5: Write out the equation for kinetic energy

K E space equals space 1 half space cross times space m space cross times space v squared

Step 6: Rearrange to make speed the subject:

v space equals space square root of fraction numerator 2 space cross times space K E over denominator m end fraction end root

Step 7: Calculate the maximum possible speed of the rollercoaster at position D

  •  At position D the rollercoaster is at ground level

  • Therefore, all the energy has been transferred from the gravitational potential to the kinetic store

  • The maximum possible speed is based on the assumption of a perfect energy transfer

v space equals space square root of fraction numerator 2 space cross times space 15 space 000 over denominator 100 end fraction end root

v space equals space 17 space straight m divided by straight s

Examiner Tips and Tricks

When the question tells you to ignore the effects of resistance (ie wasted energy transfers), this is a clue that you may need to use energy equivalency to find the missing quantity needed for your calculation.

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    Author
    Reviewer
    Leander Oates

    Author: Leander Oates

    Expertise: Physics

    Leander graduated with First-class honours in Science and Education from Sheffield Hallam University. She won the prestigious Lord Robert Winston Solomon Lipson Prize in recognition of her dedication to science and teaching excellence. After teaching and tutoring both science and maths students, Leander now brings this passion for helping young people reach their potential to her work at SME.

    Caroline Carroll

    Author: Caroline Carroll

    Expertise: Physics Subject Lead

    Caroline graduated from the University of Nottingham with a degree in Chemistry and Molecular Physics. She spent several years working as an Industrial Chemist in the automotive industry before retraining to teach. Caroline has over 12 years of experience teaching GCSE and A-level chemistry and physics. She is passionate about creating high-quality resources to help students achieve their full potential.