Transcript chap 5 energy

Chapter 5
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
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.
Work = Force|| x Distance
In this case, the distance is the
magnitude of the displacement.
Only the component of force parallel or
anitparallel to the displacement does work

F

Fy

Fx
x
W  Fx x
M= 10kg
In this case, the weight
does positive work
d=2m
Work = Mgd = (100N)(2m)
M= 10kg
Mg = 100N
Work = 200 Nm =200J
Fa
M= 10kg
In this case, the weight
does negative work
Mg = 100N
d=2m
Fa
Work = -Mgd = -(100N)(2m)
Work = -200 Nm = -200J
M= 10kg
Mg = 100N
1. WORK
Now instead of a force for how long in time
we consider a force for how long in
distance.
Work = Force x distance or W = F.d
Units - Joules (J) or ft.lb
BTU = 778 ft.lb (energy of one wooden
kitchen match)
Pushing on a wall and wall doesn’t move
(no work done on the wall)
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.
POWER
Power = Work/time or P = W/t
Units - J/s = W
550 ft.lb /s =1 hp
1 hp = 750 J/s = 750 W
1 BTU/hr = .293 W
100 W bulb = 0.1333 hp
250 hp engine = 187,500 W
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.
An object’s ability to do work by
virtue of its position.
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?
How much work does gravity do on the falling
mass?
PE = mgh
mg
h
h
Work = mgh
mg
How much energy does the mass have at the
bottom just before it hits the ground?
PE = mgh
mg
h
h
Work = mgh
mg
v
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?
How much energy does the mass have at the bottom just
before it hits the ground?
This is the kinetic energy
PE = mgh
mg
h
KE = ½ mv2
mg v
Work = mgh
V  gt
h V  Vo V
V  

t
2
2
h V  Vo V
V  

t
2
2
V  gt
h
h
2 gh
V g
g

V
V /2
V
V  2 gh
2
1
1
2
KE  mV  m2 gh 
2
2
KE  mgh  PE
Energy is conserved
Kinetic Energy Depends on Speed:
In fact, your kinetic energy is proportional
to the square of your velocity if you go twice as fast, your kinetic energy
quadruples. If you go three times as
fast,you have nine times the kinetic energy.
Kinetic Energy Depends on Mass:
Your kinetic energy is proportional to your
mass.
If mass doubles, kinetic energy doubles. If
mass triples, kinetic energy triples, too.
Difference between
momentum and Kinetic
energy
Scalar Versus Vector:
An important difference is that
momentum is a vector quantity - it
has a direction in space, and
momenta combine like forces do.
Kinetic energy is a scalar quantity it has no direction in space, and
kinetic energies combine like
"regular numbers
Dependence on Velocity:
The momentum of an object is
proportional to the object's
velocity - if you double its velocity,
you double its momentum.
The kinetic energy of an object is
proportional to the square of the
object's velocity - if you double its
velocity, you quadruple its velocity.
A Thought Experiment:
Suppose that you were captured by an
evil physicist who gave you the
following choice:
You must either:
Stand in front of a 1000 kg. truck
moving at 1 m/s, or
Stand in front of a 1 kg. frozen meatball
moving at 1000 m/s.
Truck:
Truck's momentum = mv = (1000 kg)(1 m/s)
= 1000 kg m/s
Truck's kinetic energy = 0.5 mv2 = (0.5)(1000
kg)(1 m/s)2 = 500 Joules
Meatball:
Meatball's momentum = mv =
(1 kg)(1000 m/s) = 1000 kg m/s
Meatball's kinetic energy = 0.5 mv2 =
(0.5)(1 kg)(1000 m/s)2 = 500 000 Joules
PE = 1000J
KE = 0J
PE = 800J
KE = 200J
PE = 400J
KE = 600J
PE = 0J
KE = 1000J
PE = 500J
KE = 0J
PE = 100J
KE = 400J
PE = 0J
KE = 500J
PE = 100J
KE = 400J
Machines
D
N
M
F
d
Fulcrum
Work = F
D = Nd
Mg
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
4. CONSERVATION OF
ENERGY
Galileo's inclines
Demo - Loop the loop
Energy lost due to friction is actually not
a loss; it is just a conversion.
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
A device used to multiply
forces
or simply to change the
direction of forces
Components of Machines
Lever…a bar that is free to pivot,
or turn, about a fixed point
Fulcrum….Fixed point of the
Lever
Effort Arm….Part of lever on
which the effort force is applied
Resistance arm…. Part of the
lever that exerts the resistance
force
Type I or First Class Lever
The fulcrum between the force
and the load, or between input and
output.
Type II or Second Class Lever
Reverse the position of the load
and the fulcrum.
The load is in between the fulcrum
and the effort force.
Type III or Third Class Lever
Fulcrum is at one end the load is
at the other end….Input force is
applied between them
Pulley
A Kind of a lever that can be used
to change the direction of a force.
Mechanical advantage
The ratio of resistance force to effort force
MA = Fr/Fe
Fe….effort force
Fr…..resistance force
Ideal Mechanical advantage
Workout = Workin
 Frdr = Fede
IMA = de/dr
Efficiency
Workout = Workin
 efficiency = wo/ Wi X 100%
Compound Machines
Consists of two or more simple
machines linked…..
so that the resistance force of one
machine………
becomes the effort force of the
second.
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
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