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Chapter 10 & 11
Energy & Work
Energy
• The capacity of a physical system to perform
work.
• Can be heat, kinetic or mechanical energy, light,
potential energy, electrical, or other forms.
• Energy is transformed from one form to another.
It is not created or destroyed.
Work
• Work = (force) x (distance)
• To have work, there must be:
– A force
– Movement of something by that force
Is work being done when you push against a wall?
NO! There is no movement!
Is work being done when a weightlifter
lifts a weight?
Yes – The weight is moving….
What is the weightlifter holds the weight
in place, is work being done?
NO! No movement!
Work
• Work = (Force) x (Distance)
Units: Force = Newton (N)
Distance = Meter (m)
Work = Nm = joule (J)
1 Joule of work is equal to one Newton of
force applied over the distance of 1 meter.
Potential Energy (PE)
• The stored energy an object has because of its
state, composition, or position.
– Ex: A compressed spring
• The spring is able to
bounce back due to PE
– Ex: Food, Fuels
• When atoms are rearranged, energy is released
Potential Energy (PE)
– Ex: When objects are lifted against gravity
• Gravitational Potential Energy = weight x height
= mgh
• Depends only on the weight and the vertical
displacement.
– Does not depend on the path taken!
Using the steps, ramp, or lifting
the ball directly up all produce
the same PE!
Kinetic Energy (KE)
• The energy associated with a moving object due
to its motion
– Gravitational potential energy can be transformed
into kinetic energy
Kinetic Energy = ½ mass x speed2
KE = ½ mv2
Since Speed is squared, a small change in speed
can cause a large change in KE.
KE can be positive or zero, never negative.
The Work-Energy Theorem
• The change in kinetic energy is equal to work
done.
Work = ΔKE
• If there is no change in an object’s energy, then no
work has been done.
• Energy is required to reduce the speed on an
object.
– A car is able to stop due to the work done by the
brakes.
Comparison of Kinetic Energy
and Momentum
• Momentum
– Vector Quantity
• Directional
• Able to be canceled
• Kinetic Energy
– Scalar Quantity
• Can’t be canceled
– Transform from one
form to another
– Depends on velocity
• M = mv
– Depends on velocity2
• KE = ½ mv2
Conservation of Energy
• In the absence of external work input or output,
the energy of a system remains unchanged.
Energy cannot be created or destroyed
• Energy is transformed from one form to another
The water behind a dam has potential
energy. When it flows through the
When
themovement
demolitioncan
ball or arrow
dam, the energy
of its
is released,
then the
potential
be used to power
a generator,
which
is transformed
into kinetic
can transformenergy
the energy
into
electircity energy
Power
• Power: the rate at which work is done
Remember: Work = (Force) x (Distance)
So it takes the same amount of WORK walking
up stairs as it does running….
But the POWER changes because the TIME
changes. Power is greater running up the stairs
than walking.
Power
• The rate at which energy is changed from one
form to another.
• Unit:
Work = Joule (J)
Time = second (s)
Power = J/s = watt (W)
1 Watt of power is equal to 1 joule of work done in 1
second.
Machines
• Machine: A device used to multiply a force or to
change the direction of a force
– Ex: Lever
• Changes the direction of the force
– When we push down, it pushes the object up
• (Force x Distance)input = (Force x Distance)output
– If the pivot point (fulcrum) is close to the load, then a small input
force produces a large output force
Machines
– Ex: Pulley
• Changes the direction of the force
• Does not multiply force
– Ex: Block and Tackle
• System of pulleys
• Multiplies force at the expense of distance
Efficiency
Efficiency = Work Done
Energy Used
• Using the same energy input, some machines
have a greater output
– They are more efficient!
– Energy is wasted as heat
• Typically a lever is more efficient than a pulley because
more energy is lost as heat in a pulley system