Work and Energy
More relationships between
forces, time and motion
What is energy?
Central concept in science
Energy is the mover and changer of matter
Isaac Newton didn’t think about things in terms
of energy!
We think about energy mostly in terms of what
happens when it’s transformed
Energy is defined as the capacity to do work
• Energy has units of Joules
Classifying Energy
Energy
Non-Mechanical
Mechanical Energy
Energy
Potential Energy Kinetic Energy
Heat
(Energy due to position) (Energy of motion)
Gravitational Light
Elastic Nuclear
Chemical
Conservation of Energy
Energy is neither created nor destroyed
unless there is external work input or
output on the system
Instead energy is transformed from one
form to another
Examples:
• Matches
• Car Engines
BIG IDEA: What is the system?
We saw this before when we talked
about conservation of momentum
Systems are either
• Open
• Energy is coming in or leaving, or work is being
done on the system
• Closed
• Energy is not entering or leaving the system
Conservation of Energy
BUT remember E=mc2
• Mass and energy can be converted into each
other
• Only where there are nuclear reactions going
on
• Fission
• Fusion
• Radioactive Decay
Potential Energy
Potential energy is energy stored as the
result of an object’s position
• Elastic—rubber band example
• Gravitational—dropping an apple
Units of P.E. are J
Potential Energy and Work
Work to lift an object to a height h at constant
velocity
• Applied force and force of gravity are equal and
opposite
Work Fappl h Fgrav h P.E.
Work done on the object is equal to the
potential energy of the object
• Work = P.E.
Potential Energy
P.E. Fgrav h mg h mgh
h height in m
g 10 m/s
2
Potential Energy
P.E. Fgrav h mg h mgh
h height
g 10 m/s 2
Potential Energy
Understanding the equation
P.E. Fgrav h mg h mgh
• As height is doubled, PE is doubled
• As mass is doubled, PE is doubled
Kinetic Energy
Kinetic energy is the energy of motion, where v is
velocity
KE mv 1
2
2
KE is a scalar
If mass is doubled, KE is doubled
If velocity is doubled, KE is multiplied by 4
Units of KE are Kgm2/s2=Joule
Conservation of Energy
Energy converts between potential and
kinetic in a pendulum if no friction
Ideal vs. Real World
Elastic Collisions
No energy losses
Inelastic Collisions
Energy losses
A real life inelastic collision
Watch the bear!
Inelastic in the extreme