When a force acts on an object and causes it to move through a distance, energy is transferred and work is done.
Work is only done when there is movement against an opposing force. For e.g. lifting a book off the floor to put on a table, here the movement is from the floor to the table and the opposing force is the weight of the book.
The amount of work done can be calculated by the equation:
The unit for work is Joules (J). One joule of work is done when a force of 1 Newton moves an object through a distance of 1 metre.
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In order to lift the barbell above his head the weight lifter need to apply a force which opposes the downward acting force of gravity on the mass of the barbell. The distance from the floor to above the lifters head is 2 metres.
Mass of barbell = 25 + 25 = 50kg
Weight of barbell = mass x acceleration due to gravity W = m x g
W = 50 x 9.8 = 490 Newtons
Work done = force x distance
Work done = 490 x 2 = 980 Joules
In the example above the work done by the weight lifter in lifting the weights was 980 joules. In order to do this work energy had to be transferred. 980 joules of chemical energy from food eaten by the weight lifter was transferred to 980 joules of gravitational potential energy to the barbell. Thus the amount of work done is equal to the energy transferred from one form to another.
Forms of Energy
Energy is the ability to do work.
The table below describes some of the different forms of energy.
Kinetic Energy
Energy by virtue of its motion.
This is the energy that moving objects possess.
Can be made to work when they strike another object.
Any object lifted above the ground gains gravitational potential energy.
The height the object attains gives it the potential to do work when it falls.
Examples, water in a high level reservoir, a skier on the top of a ski slope, a ski diver before jumping from a plane.
Elastic Potential Energy (Strain Energy)
The energy a material possesses when it is stretched and is put under strain.
The stored energy from the stretched material can be made to do work when released.
Examples, a stretched bow, the stretched elastic in a catapult, the tightened strings on a guitar or violin.
Thermal Energy (Heat Energy)
The energy due to the movement of atoms and molecules in a substance.
When a substance is heated up the atoms and molecules move faster and possess kinetic energy which can be used to do work.
For example heating water to produce steam to drive a turbine in electricity generation.
Chemical Energy
The energy stored in the bonds of atoms and molecules.
When the atoms and molecules undergo reactions bonds are broken and energy is released in the form of heat or kinetic energy.
Examples, digesting food, burning fuels, fuel is burnt by a rocket to do work against gravity.
Electrical Energy
Energy due to the flow of electrons.
Work can be done by the flowing electrons in an electrical circuit.
Examples, electrical appliances such as lights, ovens, motor.
Nuclear Energy
Energy stored in the nuclei of atoms.
This is the energy that holds the nucleus together.
Large amounts of energy are released when the nuclei are split or combined during nuclear reactions.
For example, radioactive uranium nuclei are split in nuclear reactors and the heat energy released is used to generate steam to drive turbines for electricity generation.
Wave Energy
Waves carry energy.
Sound is the result of the energy transferred through waves.
Radiant energy from the sun travels through electromagnetic waves.
Examples, light waves from the sun, sound waves from a talking person.
