Physics 201 Lecture 6
Work, Power, and Energy
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 displacement 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
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
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
Work can create energy in three different
ways
Work
Kinetic
Energy
Creates motion
(linear or rotation)
Potential
Energy
Displacement
from equilibrium
Heat
Energy
Temperature or
phase change
Work done on a single rigid object creates
kinetic energy
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
Weight
Gravity
Spring
Potential
Energy
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
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