Working Out Work

3. Working Out Work

Energy and Work

Revision from GCSE:

Energy - The ability to do work.
Law of Conservation of Energy - energy is neither created nor destroyed; it's converted from one form to another.
Kinetic energy - energy in a moving object.
Potential energy - energy stored up ready to do work.

Work is defined as:

work (J) = force (N) × distance moved in direction of the force (m)

In physics code:

Note:

work done is not force × displacement;
work is a scalar, although the force is a vector;
work done = energy transferred.

If the direction of the force is at an angle to the movement, the equation becomes:

Efficiency

Efficiency is defined as:

Efficiency = useful energy × 100%

total energy

Efficiency can be expressed as a fraction, or as a percentage. In practice, we never get 100 % efficiency.

Gravitational Potential Energy

Gravitational potential energy is given by:

Change in potential energy (J) = weight (N) × change in height (m)

In physics code:

Kinetic energy is given by the simple equation:

Where m is the mass and v is the velocity. Note that the energy is always positive, regardless of whether the velocity is positive or negative.

Power

Power is defined as the rate at which energy is transferred (or work is done).

Power (W) = work done (J)

time taken (s)

In physics code:

Units for power are watts (W) where 1 W = 1 joule per second (Js^-1).

There is an equation that links power and velocity:

Power (W) = Force (N) × velocity (m s^-1)

We combine the equation above with:

which gives us:

It does not take a genius to see that, since

we can write: