Work, Power, ENERGY and Simple Machines

QOTD: Write a list of 10 examples your idea of work.

Demo : create a work equation using a spring scale, string and an agenda book.

Work is done only when a force moves an object

A force acting on an object and causing it to move a distance is work

Not every force is work..if you push against the wall it does not move..that is not work!

Work = force X distance Work is measured in Joules If you pick up a bag of groceries and

walk across the room the work is picking up the groceries not the walking.

The object must move some distance as a result of your force

The force you exert must be in the same direction as the objects motion.

Ie: the groceries

You walk

5

A scientist delivers a speech to an audience of his peers.

A body builder lifts 350 pounds above his head.

A mother carries her baby from room to room.

A father pushes a baby in a carriage. A woman carries a 20 kg grocery

bag to her car?

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WHAT’S WORK?WHAT’S WORK? A scientist delivers a speech to an

audience of his peers. NoNo A body builder lifts 350 pounds

above his head. YesYes A mother carries her baby from

room to room. NoNo A father pushes a baby in a carriage.

YesYes A woman carries a 20 kg grocery

bag to her car? NoNo

Work can be determined by calculating Force used x distance moved = amount

of work

Therefore what is the formula for work? 

Work = force x distance  Joule – is the SI unit for work. Newton = force Meters = distance   Therefore if you exert: 1 Newton of force for 1 meter of

distance = 1 joule of work or 1N/m

Work is done when a force is exerted through a distance.

A student lifts a bag of books that weighs 135 N. If the bag is lifted .75 m, how much work does the student do?

F = 135 N d = .75 mW = FdW = (135 N)( .75 m)W = 101.25 J

A +24 N force is applied to an object that moves 10 min the same direction during the time that the force isapplied. How much work is done to the object?

http://www2.franciscan.edu/academic/mathsci/mathscienceintegation/MathScienceIntegation-1011.htm#item1015

Find the equation for POWER 1. Attach a string and spring scale to a large book. 2. Pull the book .5m slowly. Use a stopwatch to determine time . 3. Record the time and distance on a data table. 4. Repeat 1- 3 – but this time pull the book faster. 5. Repeat 1-3 even faster.

Force-N distance -m time- s

Power tells you how fast something is happening..how fast the work is being done

Power = work/time or Power = Force X Distance

Time Power is measured in watts (W) One watt is equal to 1 joule per second of

work divide joules/seconds

Power – the rate at which energy is transferred.

  P = W P = power Watts t W = work Joules t = time seconds   1 Watt (W) = 1 J/s  

1. m = 1500 kg t = 60 s d = 12 m  2. Equations- P = W/t W = Fd F = mg  3. Plug and chug- F = mg = (1500 kg)(9.8 m/s2) = 14,700 N   W = Fd = (14,700 N)(12 m) = 1.76 x 105 J

P = W/t = (1.76 x 105 J)/(60 s) = 2940 W  

http://www.physicsclassroom.com

What is Energy?

It turns out that energy is so fundamental, like space andtime, that there is no good answer to this question. However,just like space and time, that doesn't stop us from doing very useful calculations with Energy

We may not be able to define energy, but because it is aconserved property of nature, it's a very useful idea.

Potential Energy (PE):  Stored energy due to position Examples: rock on a cliff, battery,

food, gasoline, stretched rubber band, apple hanging in a tree

A barbell of mass "m" is lifted vertically upwards a distance "h" by an outside force. How much work does that outside force do on the barbell?

Gravitational Potential Energy

W = Fdparallel Since a = 0, Fapp = mgW = (mg) dparallel Since F and d are in the same   direction ...and dparallel = hW = (mg) h

W = mgh

Fapp

mg

Gravitational Potential Energy

But we know that in general, Eo + W = Ef.

If our barbell had no energy to begin with, Eo = 0, then W = Ef

But we just showed that we did W=mgh to lift the barbell... so mgh=Ef

The energy of a mass is increased by an amount mgh when it is raised by a height "h".

Gravitational Potential Energy

The name for this form of energy is Gravitational Potential Energy (GPE).

GPE = mgh

One important thing to note is that while changes in gravitational potential energy are important, their absolute value is not.

Gravitational Potential Energy

You can define any height to be the zero for height...and therefore the zero for GPE.

But whichever height you choose to call zero, changes in heights will result in changes of GPE. For example, the floor level can be considered zero energy or the ladder level can be zero.

0 m

0 m

0.5 m

0.5 m

Gravitational PE (GPE): Energy stored by objects that are above

the earth’s surface (objects that can fall)

 Depends on mass, acceleration and height GPE increases with height

GPE = mass gravity height

GPE = m g h = weight height

  GPE = m (kg) 9.8 m/s2 h (m)

j = 1 Nm 

9 What is the change of GPE for a 5.0 kg object which is raised from the floor to a final height of 2.0m above the floor?

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10 As an object falls, its GPE always _____.

A increases

B decreases

C stays the same

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11 What is the change of GPE for a 8.0 kg object which is lowered from an initial height of 2.0 m above the floor to a final height of 1.5m above the floor?

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12 What is the change in height of a 2.0 kg object which gained 16 J of GPE?

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GPE=mghh = GPE/mgh = 16/(2)(9.8)h = 0.82m

Kinetic Energy (KE):  Energy in the form of motion  

Depends on mass and velocity of moving object.

  Object in motion has ability to do work

http://www.youtube.com/watch?feature=player_detailpage&v=0ASLLiuejAo

Kinetic Energy

The energy an object has by virtue of its motion is called its kinetic energy. The symbol we will be using for kinetic energy is KE.

Like all forms of energy, it is measured in Joules (J).

The amount of KE an object has is given by:

KE = 1/2 mv2

KE = ½ mass velocity2

 KE = m V2

2 (j) = (kg) (m/s) 1 j = 1 kg m/s

13 As an object falls, its KE always _____.

A decreases

B increases

C stays the same.

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14 A ball falls from the top of a building to the ground below. How does the kinetic energy (KE) compare to the potential energy (PE) at the top of the building?

A KE = PE

B KE > PE

C KE < PE

D It is impossible to tell.

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15 What is the kinetic energy of a 12 kg object with a velocity of 10 m/s?

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16 What is the mass of an object which has 2400 J of KE when traveling at 6.0 m/s?

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17 A 3 kg object has 45 J of kinetic energy. What is its velocity?

18 If the speed of a car is doubled, the KE of the car is:

A quadrupled

B quartered

C halved

D doubled

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19 Which graph best represents the relationship between the KE and the velocity of an object accelerating in a straight line?

KE

v

KE

v

KE

v

KE

v

A

B

C

D

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20 The data table below lists mass and speed for 4 objects. Which 2 have the same KE?

A

A and D

B B and D

C A and C

D B and C

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Elastic Potential Energy

Energy can be stored in a spring, this energy is called Elastic Potential Energy.

Robert Hooke first observed the relationship between the force necessary to compress a spring and how much the spring was compressed.

Elastic Potential Energy

The energy imparted to the spring by this work must be stored in the Elastic Potential Energy (EPE) of the spring:

Like all forms of energy, it is measured in Joules (J).

EPE = 1/2 k x2 EPE

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22 What is the spring constant of a spring that is compressed 5 cm and has 0.65 J of elastic potential energy stored in it?

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EPE = 0.5 kx2k = EPE/0.5x2k = 0.65 / 0.5 (0.052)

k = 520 N/m

25 The same 3 kg mass compresses the same spring 2.5 cm. How much elastic potential energy is stored in the spring?

k = 1176 N/m

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The law of Conservation of Energy: Energy cannot be created or destroyed.

It may be transformed from one form into another; however, the total amount of energy in the universe remains constant. (Transformers)

Energy conversions occur without a gain or loss in energy

Energy into a system = energy out of a system

 Due to friction, energy might seem to be lost, but it has changed into thermal energy.

 .

When energy is transferred, it can transform (change form) but it still remains energy.

  Analogy:

How is energy like money?  When money is transferred from one person

or place to another it can change form (transform) but it still remains money.

Demonstrate: how bounce height of ball becomes lower and lower each time it bounces. Have students infer why this happens.

Each time the ball bounces, part of its energy is transformed into other forms of energy, such as thermal (heat) energy, sound energy and vibrations in the ground. In addition, some energy is absorbed by the ball. Therefore, it will never bounce as high as the initial drop height.

Ex: A light bulb is a device that transforms electrical energy into electromagnetic (light) energy and thermal energy

 Chemical energy (coal) heat energy

(burn to create steam) mechanical energy (steam is used to turn turbines) Electromagnetic energy (generates electricity) heat energy (blow drier, oven)

PE: 354kJKE: 0kJV: 0m/s

PE: 177kJKE: 177kJV: 26.2m/S

h=70m

Potential energy becomes Kinetic energy.

h=35m

PE: 0kJKE: 354kJV: 37.1m/S

PE: 0kJKE: 354kJV: 37.1m/S

Kinetic energy can become Potential energy.

A roller coaster is at the top of a track that is 80 m high. How fast will it be going at the bottom of the hill?

Eo + W = Ef  Eo = Ef GPE = KE mgh = 0.5mv2 v2 = 2gh v2 = 2 (9.8) 80  v =39.6 m/s 

W = 0

E0 = GPE, Ef = KE

Substitute GPE and KE equations

Solving for v yields

Conservation of Energy

A student uses a spring (with a spring constant of 180 N/m) to launch a marble vertically into the air. The mass of the marble is 0.004 kg and the spring is compressed 0.03 m. How high will the marble go?

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A student uses a spring gun (with a spring constant of 120 N/m) to launch a marble vertically into the air. The mass of the marble is 0.002 kg and the spring is compressed 0.04 m.

a)How high will the marble go?

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A roller coaster has a velocity of 25 m/s at the bottom of the first hill. How high was the hill?

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A 5 kg rock is dropped a distance of 1 m onto a spring. It compresses the spring 2 cm. What is the spring constant?

   k=245000N/m

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There are six types of simple machines: Inclined plane Wedge Screw Lever Pulley Wheel and axle http://www.youtube.com/watch?feature=pl

ayer_detailpage&v=jAPxALm9fZA

Lever

Pulley Wheel and Axle

WedgeScrewInclined Plane

The 6 Simple Machines

Lever

Pulley Wheel and Axle

WedgeScrewInclined Plane

A ramp is an example of an inclined plane

Simply put in inclined plane is a flat slanted surface

A wedge is an inclined plane that moves and is usually made up of 2 inclined planes

The screw is an inclined plane wrapped around a center bar

An inclined plane is a flat surface that is higher on one end

Inclined planes make the work of moving things easier

The mechanical advantage of an screw can be calculated by dividing the circumference by the pitch of the screw.

Pitch equals 1/ number of turns per inch.

Two inclined planes joined back to back.

Wedges are used to split things.

A lever is a rigid bar that pivots or moves around a fixed point. A seesaw is an example

Fulcrum is the fixed point of a lever A pulley is a rope, belt or chain wrapped

around a grooved wheel A pulley can change the direction of a

force or the amount of a force When you use a pulley you change the

direction of the force you are applying.

A wheel and axle is a simple machine made up of two circular objects of different sizes

The wheel is the larger object the axle is the smaller one

Bicycle is an example of a wheel and axle.. The bike wheel is the large while and the sprocket the chain wraps around is the axle

Demo: Use a ramp and 4 books and a spring scale and measure distance to move the 200g mass up vertically and horizontally on a rampCreate a data table use books as height w/ 200g hanging mass

1st Write a hypothesis –more –less- the same-work 2nd calculate the work for 1. vertically-straight up

2. up the ramp

IMA – Ideal mechanical advantage.

This is the number of times a machine is designed to multiply your effort force.

It is based on measurements of the machine.

Ignores friction

AMA – Actual mechanical advantage

This is the number of times the machine actually multiplies your effort force..

Includes the effects of friction

IMA is always greater than AMA.

Mechanical Advantage – when you increase distance you decrease force but the work remains the same.

 Machines – Multiply force redirect force- ie: pull down rope –lifts sail

work equation-  force x distance = work

Machines do not increase the amount of work. They spread out the distance so you don’t have to use the same amount of force to receive the same amount of work.

 Prove it: Work 32 J = work 32 J Force x distance force x distance 8N x 4 m 4 N x 8 m

Ideal Mechanical advantage = ratio between output force and input force or output distance and input distance without friction

If you have force information use:  Output force /Input force = MA

If you have distance information use: Input distance/output distance = MA

Mechanical advantage – multiplying force if you need 3200 N to lift a piano then use a ramp to exert 1600 N of force.

OF 3200N = 2 the ramp doubled your

IF 1600N force. Your output force is 2x your input force.

MA- is 2 no units

Mechanical Advantage – multiplying distance -you use a ramp that is 6 meters long to raise a piano 3 meters

ID- 6 meters = 2 the ramp doubled

OD 3 meters the distance

mechanical advantage of two

Write a paragraph on what you now know and did it differ from what you knew before,

Mechanical advantage to machines problem set /answers

http://library.thinkquest.org/CR0210120/Mechanical%20Advantage.html

An instrument that makes work easier is called a machine

Machines do not have to be complex electrical or gas powered deviced. Even simple objects can be a machine.

A pair of pliers would make it easier to take out a bolt so the pliers would be a machine

There are two types of work involved in using a machines: Work that goes into the machine (input) Work done by the machine (output)Work that comes out of the machine is

NEVER greater than the force that is applied to the machine or work that goes into the machine

Machines make work easier because they change either the size or the direction of the force put into the machine.

Machines multiply either the force or distance to make work easier, but never both!

The comparison of the work output to the work input is called efficiency.

The closer the amount of output is to the amount of input the more efficient the machine is.

Efficiency is measured in percent and is never more than 100%. This is because the output can never be more than the input

The lower the friction of the machine the more efficient it will be. Keeping a car engine oiled makes it work better and more efficient

Efficiency – a measure of how much work that is put into a machine is changed to useful work; answer will be a percentage.

  efficiency = Wout x 100%

Win

Win = work put into the machine

Wout = work put out by the machine

What are some factors that may make a machine inefficient?

A wooden ramp is used to push a box into the back of a truck. Mary must do 800 J of work to move the box. If there was no friction, she would only have to do 700 J of work. What is the efficiency of the machine

A rusty pulley is used to raise a pail 5 m off the ground. If the pulley was perfect, only 5000 J of work would have to be used. Because the pulley is rusty, 6500 J of work must be done. What is the efficiency?

If a machine could do 40 J of work but is only 75% efficient, what is the amount of work the machine actually does?

A windmill has an efficiency of 47%. If the wind does 250 J of work on the blades of the windmill, how much work output can the windmill do?

Wout = Frdr Fr = resistance/output force

dr = resistance/output distance

Win = Fede Fe = effort/input force

de = effort/input distance    For an ideal machine: Win = Wout

Fede = Frdr

A worker applies an effort force of 20 N to pry open a window with a resistance force of 500 N. Find the mechanical advantage of the crowbar.

Fe = 20 N Fr = 500 N MA = ?

  MA = Fr = 500 N

Fe 20 N MA = 25  

Find the effort force needed to lift a 2000 N rock, using a jack with a mechanical advantage of 10.

Fr = 2000 N MA = 10 Fe = ?

  MA = Fr / Fe

Fe = Fr / MA

Fe = (2000 N)/(10)

Fe = 200 N

www.phs.d211.org/Science/okeefenm/Okeefe/Okeefe/PhySci233/EnergyMachines/Mechanical%20Advantage.ppt –

www.cwcboe.org/gcms/teachers/apanagiotakis/Notes/Work%20&%20Power/Mechanical%20Advantage%20and%20Efficiency.ppt - Similar pages

education.jlab.org/jsat/powerpoint/0708_simple_machines_8.ppt -