1. Physics 201 Lecture 6
Work, Power, and Energy

2. Machines and the definition of work
The purpose of a mechanical machine is to multiply an input force to create a much larger force for useful work
This mechanical advantage is gained by increasing the displacement over which the force is applied
This motivates defining work as the product of displace- ment and component of force driving the displacement
Though both force and displacement are vectors, work is not – in order to account for direction we have
Is this work or weight? If they both appear in a problem, substitute mg for weight and use W for work

3. Real machines and real work
In any real machine there are frictional effects that limit its efficiency:
The rate at which work is done is called power – a more powerful machine will perform work faster
Work does not always mean what it does in English
Work is not synonymous with effort
Work does not involve time
Work can be negative

4. The energy level of a system represents its ability to do work
Work done on an internal conservative force is released when the system returns to the original configuration
This is why potential energy depends on the configuration of the system rather than the means whereby it is rearranged
But friction opposes motion – it always does negative work

5. Work can create energy in three different ways
Kinetic Energy
Creates motion
(linear or rotation)
Work
Potential Energy
Displacement
from equilibrium
Heat Energy
Temperature or phase change

6. Work done on a single rigid object creates kinetic energy

7. When driving from equilibrium, work creates potential energy
Potential energy is work against the internal forces of the system
Potential energy ultimately depends on the internal configuration of the system
Different forces will involve different values of potential energy…
Force
Potential Energy
Weight
Gravity
Spring

8. Conservation of energy
Many consider this to be the most important principle in physics
The key to use energy is to find a moment in time in which you know the total energy of the system
If there are no external forces, the total energy is always the same

9. Resistive forces destroy mechanical energy
Work done on an internal conservative force is released when the system returns to the original configuration
This is why potential energy depends on the configuration of the system rather than the means whereby it is rearranged
But friction opposes motion – it always does negative work