Transcript Chapter 20 - SFSU Physics & Astronomy

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.
Work
 An applied force
acting through a
distance parallel to
the force
 Requires movement
in same direction as
force
– Zero distance, no work
– Displacement
perpendicular to
applied force, no work
How much work?
A weight lifter lifts a barbell
weighing 1000 Newtons 1.5 meters
A weight lifter pushes on a wall with
a force of 1000 Newtons for 15 sec?
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
 The rate at which
work is done
 Units: watts (W),
horsepower (hp)
 Example: Walking
versus running
upstairs
 The “power bill” you pay for energy
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?
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?
• (a) 2 times
• (b) 3 times
• (c) 9 times
• (d) 27 times
• (e) half as far
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
Pendulum Example Problem
At what point is the kinetic energy
the highest?
– (A) At the bottom of its path
– (B) At the ends of its path
– (C) Somewhere in between
Pendulum Example Problem
At what point is the potential energy
the highest?
– (A) At the bottom of its path
– (B) At the ends of its path
– (C) Somewhere in between
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.
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
An object of mass 2 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) 900 Joules
Example Questions
A 10 lb weight is lifted 5 ft.
A 20 lb weight is lifted 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
A 1 kg mass is lifted 1 meter. The change
in potential energy of the weight is
(a) 1 N
(b) 9.8 N
(c) 1 J
(d) 9.8 J
(e) 9.8 m/s2