Using Newton’s 2nd Law to solve problems can be complicated.
You would like to find a method that is less complicated.
Using vectors makes your calculations harder.
You can use work and energy to simplify your calculation.
The law of conservation of energy is a powerful tool you can use.
The total amount of energy always remains the same.
However, some energies are difficult to measure or contain. An example of this is the conversion of
kinetic energy into heat.
Potential Energies and
Kinetic Energy are much easier
to measure. These are
known as the Mechanical
Energies.
Gravitational Potential EnergyThe gravitiational potential energy is the energy of an object due to its position in the gravitational field of the earth. The formula for the gravitational potential energy near the surface of the earth is:
E m g h
to ta l to ta lE E
If there isn’t any friction, the sum of gravitational and kinetic energy stay
the same.
m v m g h m v m g h
2 2
2 2
gkgk EEEE
Can we make a perpetual motion machine?
Well, the machine cannot violate the laws of thermodynamics?
The First Law basically says that energy or matter can neither be created nor
destroyed.
The Second Law essentially says that it is impossible to obtain a
process where the unique effect is the subtraction of a positive heat
from a reservoir and the production of a positive work.
The Third Law says that all processes cease as temperature
approaches absolute zero.
How does all of this apply to efficiency?
Efficiency is the ratio of the useful work (or useful energy) from a
machine compared to the amount of work or energy put into the
machine.
%100input
output
EE
Wind turbine
Solar Cell
Combustion Engine
Fluorescent lamps
Incandescent lights
0 10 20 30 40 50 60 70
Efficiency of Some Devices
Efficiency Questions
Find the efficiency of a block and tackle if 11000 J of energy are required to raise a 155 kg piano a distance of 6.5 m.
Find the height that the piano could be raised if the block and
tackle is 90.0% efficient.
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