Conservation of Energy

The Principle of Conservation of energy is one of the most important principles in physics. It means that we don't have to assume that energy disappears into a black hole from which it cannot be recovered. If energy is still lost to friction or heat, then using the principle of conservation of energy we can calculate those enrgy losses and start to think about how to minimise them.

The Principle of Conservation of Energy states: Energy cannot be created or destroyed. It can only be changed from one form into another.


When a ball falls, gravitational potential energy changes into kinetic energy of motion. The ball travels faster.

We can equate expressions for the two forms of energy:

When a car brakes, the car's kinetic energy is changed into heat energy in the brakes:(1)

This means that we can use measurable quantities to find things we cannot measure directly. We can use (1) to find the specific heat capacity of the material of the car brakes.

We can work out energy lost to air resistance or friction by measuring the difference between the final and initial energy. This energy is equal to F*d , the force of resistance*distance moved.

If a ball falls from height h and hits the ground with velocity v, the lost energy, the work done against air resistance,the lost energy is

Hence we can find the average force of air resistance:

Iin any process involving movement, energy is inevitably lost due to friction or air resistance. This energy ends up as heat and is usually not available to do useful work. There are exceptions. A car is heated using heat from the engine, at least part of which is generated by friction.

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