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P1.1-P1.4 (Energy and Work (To make a stationary object move, you need to…
P1.1-P1.4
Energy and Work
To make a stationary object move, you need to apply a force to the object.
When a force moves an object, energy is transferred to the object and work is done on it.
When work is done to move an object, energy supplied to the object is equal to the work done in moving the object.
Both work and energy have the unit joule, J.
The work done on an object is calculated using the equation W=F s, where:
W is the work done in joules, J.
F is the force in newtons, N.
s is the distance moved in the direction of the force in metres, m.
If an object does not move when a force is applied to it, no work is done on the object.
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Changes in Energy Stores
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Energy can be transferred by heating, waves, an electric current, or when a force moves an object.
When an object falls and gains speed, its gravitational potential energy store decreases and its kinetic energy store increases.
When a falling object hits the ground without bouncing back, its kinetic energy store is transferred by heating to the thermal energy store of the object and surroundings, and by sound waves moving away from the point of impact.
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Conservation of Energy
Energy cannot be created or destroyed. This is called the principle of conservation of energy and it applies to all energy changes.
Energy can be transferred usefully, stored, or dissipated.
A closed system is an object or group of objects in which no energy transfers take place out of or into the energy stores of the system.
However, as changes occur in a closed system, energy can be transferred to different stores within the closed system. For example:
When an object falls freely, the gravitational potential energy store of the object decreases and the kinetic energy store of an object increases.
When a person stretches an elastic band, the chemical energy store of the person's muscles decreases and the elastic potential energy store of the elastic band increases.
When a pendulum swings towards the middle, the gravitational potential energy store of the pendulum decreases and its kinetic energy store increases. As the pendulum moves away from the middle, its kinetic energy store decreases and its gravitational potential energy store increases.