Transcript Work, power and energy

Warm Up 5/2/12
What is the first law of motion?
2. Which formula is associated with the 2nd law of
motion?
3. What is the first thing to come to your mind when
you hear the word work, power and energy?
1.
Announcements
 You will have a unit test on Motion and Forces next
Friday 
 If you have any make up work you need to get it to me
by Friday, May 4th if you want it reflected on your
upcoming progress report.
What is work?
 Work is the use of force to move an object.
 In scientific terms, you only do work when you use
force to move something.
How do we calculate work?
Work= force X distance
OR
W= FD
The unit that we use to measure work is Joule (J)
or
the Newton-meter
Example 1
 How much work is done if a person lifts a barbell
weighing 500 N to a height of 2 m?
Answer
Formula for work is W=FD
W=?
F=500 N
D=2 m
W= 500/2
W=250 J
Other formulas
If you have to calculate distance:
 D= W/F
 The unit is meters
If you have to calculate force:
 F= W/d
 The unit is Newton’s
Guided Practice Problems
If you push a cart with force of 70 N for 2 M, how
much work is done?
2. If you did 200 J of work pushing a box with a force of
40 N, how far did you push the box?
3. If you get a flat tire and have to push a car 3 M and
apply a force of 3 N, how much work did you do?
4. How much force is needed to move a tractor trailer if
a machine is only allowed to use 4 J of work and it
has to move 25 M.
1.
Who or what does work?
 Just as you do work when you pick up a book or write
in your notebook, objects can also do work.
 For example, cars do work when the move down the
road, or bowling balls do work when they hit pins.
Independent Practice
1.
2.
3.
4.
5.
If you push very hard on an object but it does not
move, have you done work? Explain why or why not.
What two factors do you need to know to calculate
how much work was done in any situation?
Was work done on a book that fell from a desk to the
floor? If so what force was involved?
Work is done on a ball when a soccer player kicks it.
is the player still doing work on the ball as it is rolls
across the ground?
Tina lifted a box 0.5 M. the box weighed 25 N. how
much work did Tina do?
Answers
1.
2.
3.
4.
5.
No, the object must move for work to be done.
Force and distance.
Yes, the force of gravity.
No: the player is no longer exerting force on the ball.
W= FD
= 25 N x 0.5 m
= 12.5 J
Warm Up
5/3/12
What is the formula for work?
2. What unit do we use for work?
3. Can objects do work or just people? Be sure to
explain your answer.
1.
Announcements
 Any missing work that you want reflected on the
upcoming progress report must be given to me by
tomorrow.
 Do not hand it to me or place it on my desk….on your
way out of my room, please place it in the late work
bin.
Power
 Power is the rate at which work is done.
 Power = Work / Time
 The unit of power is the watt.
15
Check for Understanding
1.Two physics students, Ben and Bonnie, are in the
weightlifting room. Bonnie lifts the 50 kg barbell over
her head (approximately .60 m) 10 times in one
minute; Ben lifts the 50 kg barbell the same distance
over his head 10 times in 10 seconds.
Which student does the most work?
Which student delivers the most power?
Explain your answers.
16
Ben and Bonnie do
the same amount of work;
they apply the same force
to lift the same barbell the
same distance above their
heads.
Yet, Ben is the most
powerful since he does the
same work in less time.
Power and time are
inversely proportional.
17
2. How much power will it take to move a 10 kg mass
at an acceleration of 2 m/s/s a distance of 10 meters in
5 seconds? This problem requires you to use the
formulas for force, work, and power all in the correct
order.
Force=Mass x Acceleration
Force=10 x 2
Force=20 N
Work=Force x Distance
Work = 20 x 10
Work = 200 Joules
Power = Work/Time
Power = 200/5
Power = 40 watts
18
2. How much power will it take to move a 10 kg mass at
an acceleration of 2 m/s/s a distance of 10 meters in 5
seconds? This problem requires you to use the formulas
for force, work, and power all in the correct order.
Force=Mass x Acceleration
Work=Force x Distance
Power = Work/Time
19
Example 1
 An explorer uses 6000 J of work to pull his sled for 60
seconds. What power does he need?
Answer
P= w/t
P= 6000 j/60 s
P= 100 W
Practice Problems
If a conveyor belt uses 10 J to move a piece of candy a
distance of 3 m in 20 s, what is the conveyor belt’s
power?
2. An elevator uses a force of 1710 N to lift 3 people up 1
floor. Each floor is 4 m high. The elevator takes 8 s to
lift the 3 people up 2 floors. What is the elevators
power?
1.
Solutions to practice problems
P= w/t
P= 10 J/20 s
= 0.5 j/s
P= 0.5 W
2. P= w/t
W= fd
P=1710 N x 8 m
8s
P= 1710 W
1.
History of Work
Before engines and motors were invented, people
had to do things like lifting or pushing heavy loads by
hand. Using an animal could help, but what they really
needed were some clever ways to either make work
easier or faster.
24
Does this sound familiar?
 If you think about cars, they use the unit horsepower.
 Horsepower is the amount of work a horse can do in a
minute.
 We still use this unit as long ago, many people used
horses to do work.
Independent Practice
How is power related to work?
2. What do you need to know to calculate how much
energy a light bulb uses?
3. Which takes more power: using 15 N to lift a ball in 5
seconds or using 100 N to push a box 2 m in 1
minute?
1.
Answers
Power is the rate at which work is done. The faster
you do work, the greater your power.
2. Power can be measured in joules per second (watts)
or in horsepower. Examples will vary but might
include light bulbs for watts and cars for horsepower.
3. Using 15 N to lift a ball 2 m in 5 seconds.
1.
Warm Up 5/4/12
What is power?
2. How do you calculate power?
3. What unit do we use to calculate power?
1.
Announcements
 All make up work due today.
 Please put it in the late work bin on your way out today

 Check the no name folder to see if you have work
there. It will be thrown away today @ 4:15.
Energy
 This is the ability to do work.
 Work transfers energy.
There are three different types of energy:
1. Potential energy
2. Kinetic energy
3. Mechanical energy
Kinetic energy
 This is energy in motion.
 For example, when you throw a ball you transfer
energy and it moves.
 By doing work on the ball (throwing it), you give it
kinetic energy.
 Formula:
KE= 1/2mv2
Potential energy
 This is stored energy or the energy that an object has
due to its position or shape.
 For example, when you do work to lift a ball from the
ground you give the ball potential energy. Why?
Because it has the “potential” to be thrown or fall back to
the ground.
Mechanical energy
 Is the energy possessed by an object due to its position
or motion.
Or
ME= PE + KE
Example of Kinetic Energy
 What is the kinetic energy of a girl who as a mass of 40
kg and a velocity of 3 m/s?
 Remember: KE= 1/2mv2
Answer
KE= ½ mv2
= ½ x 40 kg x (3 m/s)2
=360kg
2s
=180 J
Other types of energy
 Heat energy
 Nuclear energy
 Electromagnetic energy
Independent Practice
Answer the following questions below:
1. How is water used to generate electricity?
2. Describe the pro’s and con’s of nuclear energy?
3. How can we use heat as energy?
Once you are done:
Create a cartoon that describes the different types of
energy. For each type identify the potential when
energy is considered potential and kinetic.
Stations
Station name
Thermal energy
Chemical energy
Nuclear energy
Electromagnetic
energy
What type
of energy
does it
provide?
Where do we
get it from?
Examples of
Brief
places, ways
summary of
this energy is the station
used
Warm Up 5/7/12
Explain the relationship between work and energy.
2. What is the formula for kinetic energy?
3. What is the formula for mechanical energy?
4. Briefly explain the difference between potential and
kinetic energy.
1.
Announcements
 Be sure to turn in homework 
Simple Machines
Ancient people invented simple
machines that would help them overcome
resistive forces and allow them to do the
desired work against those forces.
41
Simple Machines
The six simple machines are:







Lever
Wheel and Axle
Pulley
Inclined Plane
Wedge
Screw
42
Simple Machines
 A machine is a device that helps make work
easier to perform by accomplishing one or more
of the following functions:
 transferring a force from one place to another,
 changing the direction of a force,
 increasing the magnitude of a force, or
 increasing the distance or speed of a force.
43
Mechanical Advantage
 It is useful to think about a machine in terms of
the input force (the force you apply) and the
output force (force which is applied to the task).
 When a machine takes a small input force and
increases the magnitude of the output force, a
mechanical advantage has been produced.
44
Mechanical Advantage
 Mechanical advantage is the ratio of output force
divided by input force. If the output force is bigger
than the input force, a machine has a mechanical
advantage greater than one.
 If a machine increases an input force of 10 pounds
to an output force of 100 pounds, the machine has a
mechanical advantage (MA) of 10.
 In machines that increase distance instead of force,
the MA is the ratio of the output distance and input
distance.
 MA = output/input
45
No machine can increase
both the magnitude and the
distance of a force at the
same time.
46
The Lever
 A lever is a rigid bar
that rotates around a
fixed point called the
fulcrum.
 The bar may be either
straight or curved.
 In use, a lever has both
an effort (or applied)
force and a load
(resistant force).
47
The 3 Classes of Levers
 The class of a lever
is determined by the
location of the effort
force and the load
relative to the
fulcrum.
48
49
To find the MA of a lever, divide the output force by the input force, or
divide the length of the resistance arm by the length of the effort arm.
50
First Class Lever
 In a first-class lever the fulcrum is located at
some point between the effort and resistance
forces.
 Common examples of first-class levers include
crowbars, scissors, pliers, tin snips and seesaws.
 A first-class lever always changes the direction
of force (I.e. a downward effort force on the lever
results in an upward movement of the resistance
force).
51
Fulcrum is between EF (effort) and RF (load)
Effort moves farther than Resistance.
Multiplies EF and changes its direction
52
Second Class Lever
 With a second-class lever, the load is
located between the fulcrum and the effort
force.
 Common examples of second-class levers
include nut crackers, wheel barrows, doors,
and bottle openers.
 A second-class lever does not change the
direction of force. When the fulcrum is
located closer to the load than to the effort
force, an increase in force (mechanical
advantage) results.
53
RF (load) is between fulcrum and EF
Effort moves farther than Resistance.
Multiplies EF, but does not change its direction
54
Third Class Lever
 With a third-class lever, the effort force is
applied between the fulcrum and the resistance
force.
 Examples of third-class levers include tweezers,
hammers, and shovels.
 A third-class lever does not change the direction
of force; third-class levers always produce a gain
in speed and distance and a corresponding
decrease in force.
55
EF is between fulcrum and RF (load)
Does not multiply force
Resistance moves farther than Effort.
Multiplies the distance the effort force travels
56
Wheel and Axle
 The wheel and axle is a
simple machine
consisting of a large
wheel rigidly secured to
a smaller wheel or
shaft, called an axle.
 When either the wheel
or axle turns, the other
part also turns. One full
revolution of either part
causes one full
revolution of the other
part.
57
Pulley





A pulley consists of a grooved wheel that
turns freely in a frame called a block.
A pulley can be used to simply change
the direction of a force or to gain a
mechanical advantage, depending on how
the pulley is arranged.
A pulley is said to be a fixed pulley if it
does not rise or fall with the load being
moved. A fixed pulley changes the
direction of a force; however, it does not
create a mechanical advantage.
A moveable pulley rises and falls with the
load that is being moved. A single
moveable pulley creates a mechanical
advantage; however, it does not change
the direction of a force.
The mechanical advantage of a moveable
pulley is equal to the number of ropes
that support the moveable pulley.
58
Inclined Plane
 An inclined plane is
an even sloping
surface. The
inclined plane makes
it easier to move a
weight from a lower
to higher elevation.
59
Inclined Plane
 The mechanical
advantage of an inclined
plane is equal to the
length of the slope
divided by the height of
the inclined plane.
 While the inclined plane
produces a mechanical
advantage, it does so by
increasing the distance
through which the force
must move.
60
Although it takes less force for car A to get to the top of the ramp,
all the cars do the same amount of work.
A
B
C
61
Wedge
The wedge is a modification

of the inclined plane.
Wedges are used as either
separating or holding
devices.
 A wedge can either be
composed of one or two
inclined planes. A double
wedge can be thought of as
two inclined planes joined
together with their sloping
surfaces outward.
63
Screw
 The screw is also a
modified version of
the inclined plane.
 While this may be
somewhat difficult
to visualize, it may
help to think of the
threads of the screw
as a type of circular
ramp (or inclined
plane).
64
MA of an screw can be calculated by dividing the number of turns
per inch.
65
66
Efficiency
 We said that the input force times the distance equals the
output force times distance, or:
Input Force x Distance = Output Force x Distance
However, some output force is lost due to friction.
 The comparison of work input to work output is called
efficiency.
 No machine has 100 percent efficiency due to friction.
67
Practice Questions
1. Explain who is doing more work and why: a bricklayer
carrying bricks and placing them on the wall of a building being
constructed, or a project supervisor observing and recording
the progress of the workers from an observation booth.
2. How much work is done in pushing an object 7.0 m across a
floor with a force of 50 N and then pushing it back to its
original position? How much power is used if this work is done
in 20 sec?
3. Using a single fixed pulley, how heavy a load could you lift?
68
Practice Questions
4. Give an example of a machine in which friction is
both an advantage and a disadvantage.
5. Why is it not possible to have a machine with 100%
efficiency?
6. What is effort force? What is work input? Explain
the relationship between effort force, effort
distance, and work input.
69
Practice Questions
1. Explain who is doing more work and why: a bricklayer carrying
bricks and placing them on the wall of a building being constructed, or
a project supervisor observing and recording the progress of the
workers from an observation booth. Work is defined as a force applied
to an object, moving that object a distance in the direction of the
applied force. The bricklayer is doing more work.
2. How much work is done in pushing an object 7.0 m across a floor
with a force of 50 N and then pushing it back to its original position?
How much power is used if this work is done in 20 sec? Work = 7 m X
50 N X 2 = 700 N-m or J; Power = 700 N-m/20 sec = 35 W
3. Using a single fixed pulley, how heavy a load could you lift?Since a
fixed pulley has a mechanical advantage of one, it will only change the
direction of the force applied to it. You would be able to lift a load
equal to your own weight, minus the negative effects of friction.
70
Practice Questions
4. Give an example of a machine in which friction is both an advantage
and a disadvantage. One answer might be the use of a car jack.
Advantage of friction: It allows a car to be raised to a desired height
without slipping. Disadvantage of friction: It reduces efficiency.
5. Why is it not possible to have a machine with 100% efficiency?
Friction lowers the efficiency of a machine. Work output is always
less than work input, so an actual machine cannot be 100% efficient.
6. What is effort force? What is work input? Explain the relationship
between effort force, effort distance, and work input. The effort force
is the force applied to a machine. Work input is the work done on a
machine. The work input of a machine is equal to the effort force
times the distance over which the effort force is exerted.
71