Forces in Practice
Motion and Speed
Power and Work

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Work and Power


The following is a general equation for work done.

Work done = force x distance moved
W = fD

Force is measured in Newtons and distance in metres. Therefore 1Nm = 1 joule. Which means that work done is equivalent to energy transfered. Take the example of potential energy.

Gravitational potential energy (GPE) is the energy 'stored' in an object when it is at height, it uses the following equation:

GPE = mass x g x height
GPE = mgh

G is the force due to gravity, which is 9.81 ms-1 on earth. So when you move something up higher, kinetic energy becomes GPE, and this energy is converted back to kinetic energy when it is released.

Kinetic energy is how much energy something has when it is moving. It uses the following equation...

KE = ½ x mass x velocity2
KE = ½mv2

Isaac Newton's Laws of Motion

Isaac Newton is one of the greatest British scientists. He is most famous for coming up with the theory of gravity, after an apple fell on his head (or so the story goes). But he also came up with three laws of motion, these are...

IAn object will continue in a state of rest or uniform motion unless acted upon by an external force As an example, a ball sitting on the ground will remain stationary unless an external force acts on it (i.e. it is kicked). Or a space ship will continue to move (since there are no resistant forces in space) unless it is acted upon, so the rockets fire and it will either change direction or accelerate in the direction it is travelling.
IIThe rate of change of momentum of an object is directly proportional to the force acting on the object. And from this law we can get the equation...

force = mass x acceleration
f = ma

If the object is falling, the acceleration will be g so 9.81
IIIWhen two objects interact, the forces they exert on each other are equal and opposite Two gas particles are travelling towards each other with a force of 10N. When they collide, they will exert a 10N force on each other in the opposite direction, so they will then bounce away at exactly the same force.
A graphical demonstartion of Newton's third law of motion


Power is the amount of work done in a given amount of time and this is measured in watts, where 1 watt = 1 joule/second.

Power = work done / time

Therefore, power is the rate of transfer of energy, or the rate of doing work.