Energy
physics
Work
• 1) has to have the application of a force
• 2) movement of something by the force
• Work= force X distance
– W=Fd
• Measured in Nm which is a joule
– You can have kilojoule or megajoule just like all
other metric system numbers.
Power
• How fast we do work.
• Power= work done/ time
• The unit for power is the watt.
– Again you can have kilowatt, megawatt….
– 1 horsepower is equal to 0.75kW
• Engine rated at 134 hp is a 100-kW engine
• Question: If a forklift is replaced with a new
forklift that has twice the power, how much
greater a load can it lift in the same amount of
time? If it lifts the same load, how much faster
can it operate?
answer
• The forklift that delivers twice the power will
lift twice the load in the same time, or the
same load in half the time. Either way, the
owner of the new forklift is happy.
Mechanical energy
• Energy- the ability to do work
– Energy is measured in joules.
• Mechanical energy is the total energy in a
system.
– Kinetic + Potential = mechanical
Potential Energy
• Stored energy.
• Has the potential to do work.
• There is potential energy in fossil fuels,
batteries, and the food we eat.
• Gravitational Potential energy- potential
energy due to elevated position.
• Gpe= weight X height
– PE=mgh
Questions
• 1. how much work is done on a 100 N boulder
that you carry horizontally across a 10 m
room? How much PE does the boulder gain?
• 2. a. How much work is done on a 100 N
boulder when you lift it 1 m?
• b. what power is expended if you lift the
boulder a distance of 1m in a time of 1 s?
• c. what is the gravitational potential energy of
the boulder in the lifted position?
Answers
• 1. You do no work on the boulder moved
horizontally, because you apply no force in its
direction of motion. It has no more PE across the
room than it had initially.
• 2. a. you do 100 j of work when you lift it 1m,
since Fd=100 X 1= 100 J
• b. Power= 100/1= 100 W
• c. it depends. Rlative to its starting position, the
boulder’s PE is 100 J. Relative to some other
reference level, its PE would be some other value.
Kinetic Energy
• Energy of motion
• Kinetic energy= ½ mass X speed2
– KE= ½ mv2
– Fd= ½ mv2
– Work= KE
• Work-energy theorem= when work is done
energy changes.
– Work=∆E
Conservation of Energy
• Energy cannot be created or destroyed. It can
be transformed form one form into another,
but the total amount of energy never changes.
• Efficiency = actual mechanical advantage/
theoretical mechanical advantage.
– Efficiency= work output/ work input
Questions
• A child on a sled (total weight 500 N) is pulled up
a 10 m slope that elevates her a vertical distance
of 1m.
– A. what is the theoretical mechanical advantage of the
slope?
– If the slope is without friction, and she is pulled up the
slope at a constant speed, what will be the tension in
the rope?
– Considering the practical case where friction is
present, suppose the tension in the rope were actually
100 N. What is the actual mechanical advantage of the
slope? What would the efficiency be?
Answers
• A. 10/1= 1
• 50 N
• 500/100=5 then 5/10= .5 which equals 50%
Elastic potential Energy
• PEelastic= 1/2kx2
– k= spring constant
– x= extension or compression of the spring
measured from equilibrium.