Transcript Energy 2

Chapter 6
Energy
Energy
Universe is made up of matter
and energy.
Energy is the mover of matter.
Energy has several forms:
– Kinetic, Potential, Electrical, Heat,
etc.
Work
Now instead of a force for how long
in time we consider a force for how
long in distance.
Work = Force  Distance

W=Fd
The unit for work is the
Newton-meter which is also
called a Joule.
Questions:
How much work is done when a
weight lifter lifts a barbell weighing
1000 Newtons 1.5 meters above the
ground?
How much work is done when a
weight lifter pushes on a stationary
wall with a force of 1000 Newtons
for 15 seconds?
Power
Power is equal to the amount of
work done per unit time.
work done
Power 
time interval
The unit for power is the
Joule/second which is also called a
Watt.
Light Bulbs and Appliances
electrical energy used
Power Rating 
time interval
How much energy does a 100 Watt
light bulb use in one hour?
How about a 40 Watt light bulb?
Which bulbs shines brighter?
Mechanical Energy
When work is done on an object,
the object generally has acquired
the ability to do work.
This "ability to do work" is called
energy and it has the same units
as work….Joules.
Two Types of Mechanical Energy
– Potential Energy and Kinetic Energy
Potential Energy
The energy that is stored is called
potential energy.
Examples:
– Rubber bands
– Springs
– Bows
– Batteries
– Gravity?
Gravitational Potential Energy
PE = Weight  height
PE = m g h
Question:
– How much potential energy does a
10kg mass have relative to the ground
if it is 5 meter above the ground?
Kinetic Energy
Kinetic Energy is the energy of
motion.
Kinetic Energy = ½ mass  speed2
1
2
KE  mv
2
Question: How much kinetic
energy does a 1kg mass have if it
is moving at 10 meters/second?
Work/Energy Relationship
If you want to move something, you
have to do work.
The work done is equal to the
change in kinetic energy.
Work = DKE
Example Question
When the brakes of a car going
90 km/h are locked, how much
farther will it skid than if the
brakes lock at 30 km/h?
• Answer: 9 times
Conservation of Energy
Energy cannot be created or
destroyed; it may be transformed
from one form into another, but the
total amount of energy never
changes.
Demos
– Galileo's incline
– Bowling ball pendulum
– Loop the loop
Example Problem
A 100 kg mass is dropped from rest
from a height of 1 meter.
How much potential energy does it
have when it is released?
How much kinetic energy does it
have just before it hits the ground?
What is its speed just before
impact?
How much work could it do if it were
to strike a nail before hitting the
ground?
100 kg
KE  12 mv 2  0
PE  mgh  (100kg)(9.8m / s 2 )(1m)  980J
1 meter
100 kg
nail
100 kg
KE  12 mv 2  980 Joules
PE  mgh  0 Joules
Work Done  Force  Distace  980 Joules
Machines - An Application of
Energy Conservation
If there is no mechanical energy
losses then for a simple
machine...
work input = work output
(F
d)input = (F d)output
Examples - levers and tire jacks
Efficiency
work done
Efficiency 
energy used
Useful energy becomes wasted
energy with inefficiency.
Heat is the graveyard of useful
energy.
Comparison of Kinetic Energy
and Momentum
Kinetic energy is a scalar quantity.
Momentum is a vector quantity.
Discuss rubber bullets as
compared to lead bullets.
Example Questions
A 10 lb weight is lifted 5 ft. A 20 lb weight
is lifted 2.5 ft. Which lifting required the
most work?
(a) 10 lb weight
(b) 20 lb weight
(c) same work for each lifting
(d) not enough information is given to work
the problem
Two cars, A and B, travel as fast as they can to
the top of a hill. If their masses are equal and
they start at the same time, which one does
the most work if A gets to the top first?
(a) A
(b) B
(c) they do the same amount of work
An object of mass 6 kg is traveling at a
velocity of 30 m/s. How much total work
was required to obtain this velocity starting
from a position of rest?
(a) 180 Joules
(b) 2700 Joules
(c) 36 Joules
(d) 5 Joules
(e) 180 N
A 20 Newton weight is lifted 4 meters.
The change in potential energy of the
weight in Newton.meters is
(a) 20
(b) 24
(c) 16
(d) 80
(e) 5
Your Mission:
• Print out the study guide and define
each term.
• Read all of the “blue questions” and
answers in Chapter 6.
• Make sure that you have read this
chapter.
End of Chapter 6