3. Work, Energy + Power

What does work done mean + equation?

• Work done - the amount of energy transferred when an external force causes an object to move a distance

• W = Fs or W = Fs cosθ (work done = force x distance moved in the direction of the force)

What does joule mean?

The work done when a force of 1N moves an object through a distance of 1m

What does power mean + equation?

• Power - rate of doing work

• P = W / t (power = work done / time)

What does watt mean?

A rate of energy transfer equal to 1 joule per second

What is the other equation for power + how to derive?

P = Fv (power = force x velocity)

1. P = W / t

2. W = Fs

3. P = Fs / t

4. v = s / t

5. P = Fv

What does kinetic energy mean + equation?

• Kinetic energy - energy of a moving object

• E_k = ½ mv² (½ x mass x velocity²)

How to derive the kinetic energy equation?

1. E_k = Fx

2. F = m(v - u) / t

3. x = ½ (u + v) / t

4. E_k = (m(v - u) / t ) x (½ (u + v) / t)

5. E_k = ½ mv² - ½ mu²

6. u = 0 so E_k = ½ mv²

What does gravitational potential energy mean + equation?

• Gravitational potential energy - energy something has due to its position in a gravitational field.

• E_p = mgh (mass x gravitational field strength x height)

How to derive the gravitational potential energy equation?

1. E_p = Fx

2. F = mg

3. x = h

4. E_p = mgh

What is the principle of conservation of energy?

Energy cannot be created or destroyed, only transferred, in a closed system

What is the equation for efficiency?

Efficiency = useful output energy / total input energy x 100