Work is done only when a force makes something move, or tries to stop it moving.

Whenever work is done, energy is transferred.

The amount of work done is a measure of the amount of energy transferred.

                                     work done = energy transferred

For this reason, work and energy are measured in the same units, joules (J).

The amount of work done depends on the size of the force applied and the distance something moves.

work done    =   force applied    x   distance moved in direction of force
    (joule, J)            (newton, N)                             (metre, m)

If a person who weighs 500 N climbs a flight of stairs that have a vertical height of 5 m, the amount of work the person does is:

                                     work done = 500 x 5
                                                       = 2 500 J (or 2,5 kJ)

Sometimes the mass of an object is given, instead of its weight.

To convert mass to weight use the following equation:

              weight     =          mass              x          gravitational field strength
          (newton, N)         (kilogram, kg)                     (newton/kilogram, N/kg)

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Power

Power is a measure of how fast work is done, or how fast energy is transferred.

The greater the power, the more energy is transferred in a given time.

Power is measured in watts (W).

    power (watt, W) = unit of work done (joule, J)
                                     time taken (second,s)

1 watt is equivalent to 1 joule of energy being transferred every second.

Therefore 1 watt is equivalent to 1 J/s

If a weightlifter raises a load of 1 500 N to a height of 2 m in 3 seconds, the power developed is:

                power = 1 500 x 2
                                    3
                power =    3 000
                                    3

                           = 1 000 W (or 1 kW)

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Gravitational potential energy is the energy stored in an object when it is raised above the ground against the force of gravity.

The higher the object is raised, the more gravitational potential energy it stores.

change in gravitational potential energy = weight x change in vertical height
                          (joule, J)                                 (newton, N)                 (metre, m)

If someone lifts a 100 N baby, 2 m off the floor, it will gain gravitational potential energy:

               Gain in gravitational potential energy = 100 x 2
                                                                          = 200 J

An object at ground level has zero gravitational potential energy.

Elastic potential energy is the energy stored in an elastic object when work is done to change its shape.

For example, a stretched catapult has elastic potential energy.

When the catapult is released, its elastic potential energy is transferred into kinetic (movement) energy.

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Kinetic Energy

Kinetic energy is the energy an object has because of its movement.

An object has more kinetic energy if it has a greater mass, or a faster speed.

                     kinetic energy        =     ½     x      mass         x               speed²
                        (joule, J)                                   (kilogram, kg)      [(metre/second)², m/s²]

The kinetic energy of a 70 kg man moving at 2 m/s is:

                     kinetic energy = ½ x 70 x 2²
                                            = 35 x 4
                                            = 140 J

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