Apr 18, 2023 General Science Chapter 13 2
Work
Everyday concept of work.Scientific definition: Work is the
transfer of energy through motion. A force applied, in which the object
moves, to an object in the direction that the object moves.
Apr 18, 2023 General Science Chapter 13 3
Work
In order for work to take place, a force must be exerted through a distance.
In order for work to be done, there has to be motion, and the motion has to be in the direction of the applied force.
If there is no motion, no work will be done
Apr 18, 2023 General Science Chapter 13 4
Recall “work” lab
Did you do more work lifting the books to shoulder level or over your head?
Greater distance means more workDid you do more work lifting 2 books or 4
books?Greater force means more work
Apr 18, 2023 General Science Chapter 13 5
Work Equation
Work, like energy, is measured in joules. 1 J = 1 N ∙ m.
distanceforceWork ×=
dFW ×=
Apr 18, 2023 General Science Chapter 13 6
Work and Energy
Work is the transfer of energy through motion.
When 1 J of work is done on an object, 1 J of energy has been transferred to the object.
Apr 18, 2023 General Science Chapter 13 7
Example
A student’s backpack weighs 10 N. She lifts it from the floor to a shelf 1.5 m high. How much work is done on the backpack?
Force, F = 10 NDistance, d = 1.5 mWork = (Force)(Distance)Work = (10 N)(1.5 m)Work = 15 N ∙m = 15 J
Apr 18, 2023 General Science Chapter 13 8
Example #2
A dancer lifts a 400-N ballerina overhead a distance of 1.4 m and holds her there for several seconds. How much work is done on the ballerina?
Work = (400 N)(1.4 m)Work = 560 J
Note that the time was not important for us to determine the work done.
Apr 18, 2023 General Science Chapter 13 9
Example #3
A carpenter lifts a 45-kg beam 1.2 m high. How much work is done on the beam?
Remember that weight equals mass times acceleration due to gravity.
Weight = (45 kg)(9.8 m/s2) = 441 NWork = (441 N)(1.2 m)Work = 529.2 J
Power
Apr 18, 2023 General Science Chapter 13 10
The rate at which work is done. How much work is done in a given
amount of time The ratio of work to time
€
Power =Work
time
t
WP =
Watts
Apr 18, 2023 General Science Chapter 13 11
Power is measured in Watts, named after James Watt, who helped develop the steam engine.
1 W = 1 J/s Very small unit, so we often use kW. 745.6 Watts = 1 horsepower
Example
Apr 18, 2023 General Science Chapter 13 12
A figure skater lifts his partner, who weighs 450 N, 1 meter in 3 seconds. How much power is required?
t
dF
t
WP
⋅==
s 3
m 1N 450 ⋅=P
W150=P
You try
Apr 18, 2023 General Science Chapter 13 13
A 25 000 N elevator rises 30.0 m in 60.0 s. How much power is required? Express your answer in kW.
s 60
m 0.30N 25000 ⋅=P
kW 12.5 W12500 ==P
Making Connections
Apr 18, 2023 General Science Chapter 13 14
4.184 Joules = 1 calorieJoules are units for energy and work
1 Calorie = 1000 caloriesA Calorie is used for foods, so if a candy bar
has 250 Calories it is the same as 250000 calories.
Example #1
Apr 18, 2023 General Science Chapter 13 15
Tommy eats a candy bar that has 230 Calories. How many Joules is that?
1 Calorie = 1000 calories 1 calorie = 4.184 Joules
so1 Cal = 4184 Joules therefore
230 Calories = 962320 Joules
Example #1 continued
Apr 18, 2023 General Science Chapter 13 16
How much Power can be produced with the 230 Calorie candy bar in 1 hour?
230 Cal = 962320 JoulesUse the equation below
t
WP =
Example #1 continued
Apr 18, 2023 General Science Chapter 13 17€
P =962320 J
3600 s
€
P = 267.3 W
€
P = 267.3 W or 0.3585 hp
Discussion #1
Apr 18, 2023 General Science Chapter 13 18
Define work and what are the SI units?What units are used to measure Power?Why is the unit kW used more often than
W?What is the conversion factor for
horsepower to Watts?
Apr 18, 2023 General Science Chapter 13 19
Machine
A device that makes work easierBy using a machine you DO NOT DO
LESS work.It just makes it seems easier to do the
work.
Apr 18, 2023 General Science Chapter 13 20
Work and machines
Work is made easier by doing 1 of 3 thingsChanges the size of the input forceChanges the direction of the forceChanges both the size and the direction
of the forceOpening a paint can with a screwdriver
• Changes size – you can use less force• Changes direction
Apr 18, 2023 General Science Chapter 13 21
Forces and machines
Effort force (Fe) – applied to the machineThe force you exertAlso called input force
Resistance force (Fr) – applied by the machine to overcome gravity or frictionThe force the machine exertsAlso called output force
Apr 18, 2023 General Science Chapter 13 22
Work and machines
Work input (Win) – work done on the machineEffort force times distance it movesWin = Fe X de
Work output (Wout) – work done by the machineResistance force times distance it movesWout = Fr X dr
Apr 18, 2023 General Science Chapter 13 23
Mechanical advantage
The number of times a machine multiplies the effort force
The ratio of output to input. (usually a force)
e
r
F
FMA=
€
MA =dedrordindout
Apr 18, 2023 General Science Chapter 13 24
Mechanical advantage
Can be greater than 1Opening paint can
Can be equal to 1Raising blinds
Can be less than 1Hitting a baseball
Apr 18, 2023 General Science Chapter 13 25
Example
A worker applies an effort force of 10 N to pry open a window that has a resistance of 500 N. What is the mechanical advantage of the crowbar?
Fr = 500 N Fe = 10 N
N 10
N 500==
e
r
FF
MA 50=MA
Apr 18, 2023 General Science Chapter 13 26
You try
A jack is used to lift a 2000-N rock. The effort force is 200 N. Find the mechanical advantage.
N 200
N 2000==
e
r
FF
MA 10=MA
Apr 18, 2023 General Science Chapter 13 27
Discuss #2
A _____________ is a device that makes work easier.
What are the 3 ways a machine can make work easier?
What do we call the force applied to a machine?
What do we call the force applied by a machine?
Apr 18, 2023 General Science Chapter 13 28
Discussion #2
What is mechanical advantage?What does it mean when the MA value is
equal to 1?How do we calculate MA?
Apr 18, 2023 General Science Chapter 13 29
Simple machine
A device that does work with only one movement
There are six types Levers Inclined PlanePulley WedgeWheel & Axle Screw
Ideal Mechanical Advantage
The ratio of output to input (usually a force) disregarding friction and gravity.
When the output work = the input work.Machine would be 100% efficient, which is
impossible.
04/18/23 General Science Chapter 13 30
Apr 18, 2023 General Science Chapter 13 32
Definitions
A lever is a bar that is free to pivot, or turn, about a fixed point.
A fulcrum is the fixed point of a lever.The effort arm is the part of the lever on
which the force is applied.The resistance arm is the part of the
lever that exerts the resistance.
Apr 18, 2023 General Science Chapter 13 33
Lever
fulcrumResistance arm Effort arm
Effort forceResistance force
Apr 18, 2023 General Science Chapter 13 34
Mechanical advantage
Review, we learned that
We can also use for levers
€
IMA =length of effort arm
length of resistance arm=LeLr
e
r
F
FMA=
Apr 18, 2023 General Science Chapter 13 35
You try
You use a crowbar 160 cm long as a lever to lift a large rock. The rock is 20 cm from the fulcrum. You push down on the other end of the crowbar.
What is the length of the effort arm?The resistance arm?What is the IMA of the lever?
Apr 18, 2023 General Science Chapter 13 36
First class levers
The fulcrum is in the middleSeesawcrowbar
fulcrum
Resistance arm Effort arm
Effort force
Apr 18, 2023 General Science Chapter 13 37
Second class levers
The resistance is in the middlewheelbarrownutcracker
fulcrum
Resistance arm Effort arm
Effort force
Apr 18, 2023 General Science Chapter 13 38
Third class levers
The effort is in the middleBaseball batbroom
fulcrum
Resistance arm
Effort arm
Effort force
Apr 18, 2023 General Science Chapter 13 39
Discuss #3
What is a lever?What is a fulcrum?What is the effort arm?What is the resistance arm?
Apr 18, 2023 General Science Chapter 13 40
Discussion #3
What are the 3 types of levers?What is an example of each type of lever?Which type usually has a IMA value < 1?
Apr 18, 2023 General Science Chapter 13 41
Pulleys
Pulley – grooved wheel with a rope or chain running along the groove
Acts like a leverThe two ends of the rope are the effort arm and
the resistance armThe wheel acts like the fulcrum
Apr 18, 2023 General Science Chapter 13 42
Fixed pulley
Attached to something that doesn’t moveChange the direction of a forceIMA of 1
Le
Lr
Fr Fe
Apr 18, 2023 General Science Chapter 13 43
Movable pulley
Attached to the object being movedIMA greater than 1Effort distance must be greater than
resistance distance
Le
Lr
Fr
Fe
Apr 18, 2023 General Science Chapter 13 44
Block and tackle
System of fixed and movable pulleysHas IMA equal to the number of ropes
that support the resistance weightCount every rope coming off the movable
pulleys that supports or moves the resistance force.
Apr 18, 2023 General Science Chapter 13 47
Wheel and axle
Consists of two wheels of different sizes that rotate togetherThe effort force is usually applied to the large
wheelThe small wheel, or axle, exerts the resistance
force
Examples: doorknob, water faucet, gears, pencil sharpener
Apr 18, 2023 General Science Chapter 13 48
Wheel and axle
Can be thought of as a lever attached to a shaftRadius of wheel is effort armRadius of axle is resistance armCenter of axle is fulcrum
€
IMA =radius of wheel
radius of axle=rwra
Apr 18, 2023 General Science Chapter 13 49
Inclined plane
A rampLifting something along an inclined plane
means you cover more distance than lifting it straight up, but you get to use a smaller force
€
IMA =effort distance
resistance distance=
length
height=l
h
Apr 18, 2023 General Science Chapter 13 50
Screw
An inclined plane wrapped in a spiral around a cylindrical post.
As you drive in a screw, the inclined plane slides through the wood.
Apr 18, 2023 General Science Chapter 13 51
Wedge
An inclined plane with one or two sloping sides
ExamplesChiselsKnivesAxe blades
The material stays in place while the wedge moves through it.
Apr 18, 2023 General Science Chapter 13 52
Wedge
€
IMA =effort distance
resistance distance=
Thickness
side=T
S
Thickness, T
Side, S
Apr 18, 2023 General Science Chapter 13 53
Variations
All six kinds of simple machines are variations of two basic machinesThe leverThe inclined plane
Apr 18, 2023 General Science Chapter 13 54
Compound Machine
A machine that is made up of 2 or more simple machines.
Examples of compound machinesFishing rod, pencil sharpener, an axe
Apr 18, 2023 General Science Chapter 13 55
Discuss #4
What kind of simple machine is a ramp?What is an inclined plane wrapped in a
spiral around a cylindrical post?What kind of simple machine are chisels,
knives, and axes?
Apr 18, 2023 General Science Chapter 13 56
Discussion #4
What type of pulleys have a MA = 1?What is the difference between a fixed
pulley and a movable pulley?What is a block and tackle?What two groups can simple machines be
broken into?What is a compound machine?
Apr 18, 2023 General Science Chapter 13 57
Energy
Scientific definition: Energy is the ability to cause change.
Ability to do workAny sample of matter has energy if it can
produce a change in itself or in its surroundings.
Energy comes in many forms and will be classified as either Kinetic or Potential.
Apr 18, 2023 General Science Chapter 13 58
Energy continued
Kinetic FormsRadiant (solar), thermal, electrical, wind, sound
Potential FormsGravitational, mechanical, chemical, and
nuclear
Energy is measured in joules (J).Named after a British scientist.
Apr 18, 2023 General Science Chapter 13 59
Kinetic energy
Energy in the form of motionAmount depends on the mass and
velocity of the object.Greater mass at the same velocity OR
greater velocity with the same mass will have greater kinetic energy
KE = ½mv2
5 Types (STEWS or SHEWS)
Apr 18, 2023 General Science Chapter 13 60
Solar Energy (Radiant)
Electromagnetic energy that travels in transverse waves.
Energy from the sun
Apr 18, 2023 General Science Chapter 13 61
Thermal Energy
Heat energy, the internal energy in a substance.
Caused by the vibration and movement of atoms/molecules within substances.Geothermal energy is a good example of this
type of energy.
Apr 18, 2023 General Science Chapter 13 62
Electrical Energy
Energy produced by the movement of electrons.Lightning and electricity are good examples of
this form of energy.
Apr 18, 2023 General Science Chapter 13 63
Wind Energy (Motion)
Energy produced from the movement of objects from one place to another.
Do not need to see this movement, we just know there is a change in position.Wind and some forms of hydropower are good
examples of this form of energy.
Apr 18, 2023 General Science Chapter 13 64
Sound Energy
Movement of energy through substances using longitudinal or compressional waves.
Obviously this is how we hear “things”A compressional wave is like the
movement of an inch worm or an accordion.
Apr 18, 2023 General Science Chapter 13 65
Potential energy
Stored energyDepends on its position/condition/height,
mass and gravity4 Types (GECN)
PE = mghm = mass, g = gravity, h = height
Apr 18, 2023 General Science Chapter 13 66
Gravitational Energy (Hydro)
Potential energy of an object due to height above the earth’s surface.
The higher the object is, the more potential energy it has.
Apr 18, 2023 General Science Chapter 13 67
Elastic Potential Energy (Stored)
Energy stored in a spring or rubber band or anything else that stretches.
The farther it is stretched, the greater its potential energy.
Energy based on the position
Apr 18, 2023 General Science Chapter 13 68
Chemical Energy
The energy stored in foods, fuels, and batteries.
There must be a chemical reaction to get the energy out.
Apr 18, 2023 General Science Chapter 13 69
Nuclear Energy
Energy stored in the nucleus of an atom.Fusion and Fission are two examples
Mechanical Energy
The sum of potential and kinetic energy in a system is called mechanical energy.
Think about a roller coaster or bungee jumping.
Apr 18, 2023 General Science Chapter 13 70
€
ME = PE +KE
Apr 18, 2023 General Science Chapter 13 71
Discuss #5 Energy Review
Define energyWhat units are used to describe energyWhat are the 2 main forms of energyList 3 of the 4 subsets of stored energyList 3 of the 5 subsets of moving energyWhat is Mechanical energy?
Apr 18, 2023 General Science Chapter 13 72
Discussion #5
Why is the first hill of a roller coaster ride the highest?
Where would a roller coaster be moving fastest?
Apr 18, 2023 General Science Chapter 13 73
Discussion #5b
Why can you not travel in a circular loop when on a roller coaster?
Is it possible for the second hill to be taller than the first hill? Explain why?
When does a coaster have the most PE?When does a coaster have the most KE?
Apr 18, 2023 General Science Chapter 13 74
Conservation of Energy
Energy cannot be created or destroyed but it can change from one form to another.
Example – SwingWhy does it stop?
Apr 18, 2023 General Science Chapter 13 75
Conservation of energy
You can never get more work out than you put in
If force increases, distance must decrease.
outin WW =
rree dFdF ×=×
Apr 18, 2023 General Science Chapter 13 76
Internal energy
The total energy of all the particles that make up a sample of matter.
Includes both kinetic and potential energy of the particles.
The more mass a material has, the more internal energy it has.
Apr 18, 2023 General Science Chapter 13 77
Internal energy
Different materials have different internal energies even when they have the same mass and temperature.
This is because the particles in the materials are arranged differently.
Apr 18, 2023 General Science Chapter 13 78
Separate energies
Internal energy of a material depends on the total energy of its particles.
Mechanical energy (kinetic and potential) of the material itself has no effect on internal energy.
1st Law of Thermodynamics
The net change in energy equals the energy transferred as work and heat.
Q = HeatW = WorkΔU = Internal energy
Apr 18, 2023 General Science Chapter 13 79
€
ΔU =W +Q
Apr 18, 2023 General Science Chapter 13 80
Efficiency
Measure of how much of the work put into a machine is changed to useful work put out by the machine.
%100⋅=in
out
WW
efficiency
%100⋅⋅⋅
=ee
rr
dFdF
efficiency
€
efficiency =IMA
MA⋅100%
Apr 18, 2023 General Science Chapter 13 81
Efficiency
Can it ever be more than 100%?How can we increase efficiency?
Apr 18, 2023 General Science Chapter 13 82
example
A worker pushes a 1500 N chair up an inclined plane that is 4.0 m long and 1.0 m high. The worker exerts a force of 500 N. What is the efficiency of the inclined plane?
%100⋅⋅⋅
=ee
rr
dFdF
efficiency
%100m 0.4N 500
m 0.1N 1500⋅
⋅⋅
=efficiency
%75%1002000
1500=⋅=efficiency
1 m4 m