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Science 8: Unit D


Science 8: Unit D Topic 3: Work, Kinetic Energy, and Potential Energy Potential Energy Energy that is stored in an object because of its position or special condition. – PowerPoint PPT presentation

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Title: Science 8: Unit D

Science 8 Unit D
  • Topic 3 Work, Kinetic Energy, and Potential

Potential Energy
  • Energy that is stored in an object because of its
    position or special condition.
  • Eg. the arrow in the photo has PE because if the
    bow is released the arrow will fly out with a
    large amount of kinetic energy.

Gravitational Potential Energy
  • A form of PE Energy is stored in the object
    because of its position above the ground (ie. it
    can fall). If the object is released or falls its
    GPE will change to KE.
  • Eg. the people in the photo have GPE compared to
    river below.

Kinetic Energy
  • The energy of motion.
  • Any object with mass that is moving with an speed
    has this energy.

The Connection Between Work and Energy
  • Remember Any time work is done energy is changed
    or moved from an object/area to another.
  • If a can of paint is lifted to a shelf, work is
    done and its GPE is increased (by the same
  • If a sports car travelling with a KE of 10000J
    hits a pedestrian, it will do work on the
    pedestrian by increasing his KE (and also
    breaking more than a few bones).

Bullet (with KE) doing work on an Apple.
The Formulas
  • Work, GPE, and KE are all measured in the same
    units Joules (J).
  • KE 0.5XmassXspeedXspeed or ½XmXs2
  • Mass is in kg
  • Speed is in metres/second (m/s)
  • GPE massXgXheight or mgh
  • Mass is in kg
  • G 9.8 N/kg
  • h is in metres

Example Problems
  • Eg1. Tommy is a construction worker (m65 kg)
    working on the new Freedom Tower in NYC. If hes
    sitting on a beam 80 m above the ground, whats
    his GPE and KE relative to the ground?
  • GPE mgh 65kgX9.8N/kg80m 129360 J or 129
  • KE 0 J as hes not moving.

Examples Contd
  • Eg2. Tommy drops an apple (m 0.05 kg)and watches
    it fall. If is falling at a speed of 30 m/s and
    is 15 m above the ground, what is its GPE and KE?
  • KE 0.5XmXsXs 0.5 X 0.05 kg X 30 m/s X 30 m/s
    22.5 J
  • GPE mgh 0.05 kg X 9.8 N/kg X 15 m 7.4 J

Efficiency of Machines
  • Work is the transfer of energy from a source to a
    destination. The job of a machine is to transfer
    the work from source to destination. How well it
    does this is its efficiency.
  • Efficiency Workoutput/Workinput100

Transformation and Transmission
  • Transmission - The moving of energy from one
    place to another without changing the type of
    energy it is.
  • Eg. Moving energy from the pedal of the bike to
    the wheel of the bike is transmission. Chemical
    energy in your body to moving energy of your foot
    is not transmission.
  • Transformation the changing of the energys
    form (eg. light to heat, electricity to sound)