May 15General Science Chapter 131 Work & Energy Chapter 13

Apr 18, 2023 General Science Chapter 13 1

Work & Energy

Chapter 13


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.


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


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


Work Equation

Work, like energy, is measured in joules. 1 J = 1 N ∙ m.

distanceforceWork ×=

dFW ×=


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.


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


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.


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


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


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


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


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


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


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


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


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?


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.


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


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


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


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



Can be greater than 1Opening paint can

Can be equal to 1Raising blinds

Can be less than 1Hitting a baseball


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


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


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?


Discussion #2

What is mechanical advantage?What does it mean when the MA value is

equal to 1?How do we calculate MA?


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


Levers

ExamplesCrowbarsSeesawsBaseball bat


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.


Lever

fulcrumResistance arm Effort arm

Effort forceResistance force



Review, we learned that

We can also use for levers

€

IMA =length of effort arm

length of resistance arm=LeLr

e

r

F

FMA=


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?


First class levers

The fulcrum is in the middleSeesawcrowbar

fulcrum

Resistance arm Effort arm

Effort force


Second class levers

The resistance is in the middlewheelbarrownutcracker

fulcrum

Resistance arm Effort arm

Effort force


Third class levers

The effort is in the middleBaseball batbroom

fulcrum

Resistance arm

Effort arm

Effort force


Discuss #3

What is a lever?What is a fulcrum?What is the effort arm?What is the resistance arm?


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?


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


Fixed pulley

Attached to something that doesn’t moveChange the direction of a forceIMA of 1

Le

Lr

Fr Fe


Movable pulley

Attached to the object being movedIMA greater than 1Effort distance must be greater than

resistance distance

Le

Lr

Fr

Fe


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.


Examples of Block & Tackle


Example of Block & Tackle #2


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


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


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


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.


Wedge

An inclined plane with one or two sloping sides

ExamplesChiselsKnivesAxe blades

The material stays in place while the wedge moves through it.


Wedge

€

IMA =effort distance

resistance distance=

Thickness

side=T

S

Thickness, T

Side, S


Variations

All six kinds of simple machines are variations of two basic machinesThe leverThe inclined plane


Compound Machine

A machine that is made up of 2 or more simple machines.

Examples of compound machinesFishing rod, pencil sharpener, an axe


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?


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?


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.


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.


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)


Solar Energy (Radiant)

Electromagnetic energy that travels in transverse waves.

Energy from the sun


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.


Electrical Energy

Energy produced by the movement of electrons.Lightning and electricity are good examples of

this form of energy.


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.


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.


Potential energy

Stored energyDepends on its position/condition/height,

mass and gravity4 Types (GECN)

PE = mghm = mass, g = gravity, h = height


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.


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


Chemical Energy

The energy stored in foods, fuels, and batteries.

There must be a chemical reaction to get the energy out.


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.


€

ME = PE +KE


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?


Discussion #5

Why is the first hill of a roller coaster ride the highest?

Where would a roller coaster be moving fastest?


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?


Conservation of Energy

Energy cannot be created or destroyed but it can change from one form to another.

Example – SwingWhy does it stop?


Conservation of energy

You can never get more work out than you put in

If force increases, distance must decrease.

outin WW =

rree dFdF ×=×


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.


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.


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


€

ΔU =W +Q


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%


Efficiency

Can it ever be more than 100%?How can we increase efficiency?


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


You try

Using a fixed pulley, you pull the rope down 1.0 m with a force of 72 N. A 65-N object is raised 1.0 m. What is the efficiency of the pulley?

%90%10072

65=⋅=efficiency

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May 15General Science Chapter 131 Work & Energy Chapter 13