Transcript CPO Chapter 5 Work Energy Power

5.1 Work
Work – In science, work is defined as
force times distance.
Joule – The unit of work.
Efficiency and Loss of Input
• When most or all of the work input into a
machine is transferred to output, it is said
to be efficient. In real machines there is
always less output than input. Friction
and other forces use up some of the work
energy.
• Efficiency is measured in a ratio of work
output to work input. Divide output by
input. Convert to a percent by multiplying
by 100.
What is the efficiency of this machine?
%Efficiency = Work out (100) = 3 J (100) = 75%
Work in
4J
Power
Power – The rate at which work is done
is called power.
Watt – The unit of power. It is equal to one joule
of work per second.
5.2 Energy Conservation
• Energy is the
ability to do work.
It is the ability to
do work so it is
measured in
joules as well.
Law of Conservation of energy
• Energy can never be created or destroyed, just
transformed from one form into another. The sum of
the potential and kinetic energy will always be the
same, although some of the energy may be converted
to frictional heat, but it is not destroyed.
Potential Energy
Potential energy comes from the
position of an object relative to
the Earth. It is stored
energy. Potential energy is
not “used” until the object is
moved to a lower position.
Kinetic Energy
The energy of motion. An object in motion
stores energy in this motion. It is the energy
of motion. It increases with speed. It also
can increase with mass.
Conservation of Energy
The total energy of a system can be calculated by
using the Potential energy equation for the
object at it’s highest position.
EP = mgh = 10kg (9.8 m/s2) 30m
m = 10 kg
m = 10 kg
g = 9.8 m/s2
h = 30 m
30 m
EP= 2940 J
This is the total energy that
the ball will ever have.
What about if the ball fell
one step? What would
happen to the energy?
Can you find the potential
energy of the ball on this step?
EP= 10kg (9.8 m/s2) 20m
m = 10 kg
g = 9.8
EP = mgh
m/s2
EP= 1960 J
h = 20 m
What was the
total energy of
this system?
2940 J
So what happened to the rest of the
energy? It was expressed as kinetic energy as
m = 10 kg
the ball fell.
How much kinetic energy?
Conservation of energy states the total
energy of the system should equal all the
parts of the energy. ETotal= EP + EK
30 m
20 m
2940 J = 1960 J + EK
EK = 980 J