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Work, Energy and Power
Work and Energy  
When a force moves in the direction along which it acts, work is done by that force.  
 
So, one might ask the question... what exactly is work?  
   
Work is the process of converting energy from one form to another (electrical energy to thermal energy, chemical energy to kinetic energy etc)  
   
OK, so, what exactly is energy?  
   
Well... energy is what you need if you want to do work...!  
(Slightly more formal sounding statement: Energy is the ability to do work.)  
   
If these two inter-dependent definitions seem less than satisfactory, look at it from a "practical" point of view. Do we know what we mean by energy?  
   
Yes; energy is the useful thing we get from electricity, chemical reactions like burning, motive force from the the wind (kinetic energy), potential for water stored behind a dam to give us electricity (gravitational potential energy) etc  
   
In other words, the two terms, work done and energy converted, mean exactly the same thing.  
The quantity of work done (or energy converted) by a force, when the force moves, depends on two obvious factors:
- the distance moved (in the direction along which the force acts) and
- the magnitude (strength) of the force
 
   
To convince yourself of this, consider first climbing up 20 stairs and then later climbing up 200 stairs; in which case are you more tired?  
Greater distance; you will be more tired.  
Similarly, consider first climbing up those stairs carrying nothing and then climbing up those same stairs carrying a 50kg sack of potatoes* on your back.  
Bigger force needed; again, more tiring.  
   
We use this idea to define the quantity of work done in a given situation by the following equation:  
work done = force distance moved in the direction of the force  
 
note that, although both force and displacement (distance moved in a specified direction) are vector quantities, work (energy) is a scalar quantity. The above equation is said to represent a scalar product of two vectors.  
   
The equation above also leads us to a unit for measuring quantities of work, the Newtonmetre.  
   
However, in honour of a scientist who did a lot of work on work (sorry about that), James Prescott Joule (1818 - 1889), 1Newtonmetre is called 1Joule  
   
1Joule is the quantity of work done when a force of 1N moves a direction of 1m along its own line of action.  
If the direction of the displacement is not the same as the direction of the force, we use the component of the force which is parallel to the displacement.  
 
For example, when a body is caused to accelerate down an inclined plane by the force of gravity, as shown here.  
 
The magnitude of the component of the force of gravity acting parallel to the displacement is given by the magnitude of the force multiplied by the cosine of the angle between the force and the displacement.  
 
We can therefore state the more general equation:  
 
One of the most useful facts about energy is that the total amount of energy in the universe is thought to be constant.  
This idea is expressed in the law of conservation of energy:  
The energy in any closed system is never lost but can be changed from one form to another.  
   
This law can help us simplify many calculations/predictions in physics, click here for an example.  
   
Examples of energy conversions:  
- A battery converts chemical energy into electrical energy  
- An electric motor lifting an elevator converts electrical energy into gravitational potential energy  
- A nuclear reactor converts nuclear energy into thermal energy.  
- When a moving vehicle uses its brakes to stop, the kinetic energy of the vehicle is converted to thermal energy in the brakes.  
 (This is an interesting case because thermal energy is just kinetic energy but possessed by many randomly moving particles...)  
   
Power  
Consider two car motors.  
One might be described as being more powerful than the other.  
Both motors are capable of doing the same amount of work but the more powerful motor can do the work more quickly.  
This consideration leads us to the following definition of the word power:  
Power is the rate of doing work (or converting energy) or  
Power is the work done (or energy converted) per unit time  
 
the unit of power is therefore the Joule per second, Js-1  
However, a power of 1Js-1 is called 1Watt (1W) after James Watt an 18th century mechanical engineer who did work on steam engines.  
   
Combining this equation with the definition of work we obtain another useful equation:  
 
Click here for power calculation example  
   
   
*ok, so it doesn't specifically have to be potatoes... you get the point!  
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