Work, Energy & Power

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What’s work?• 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 km grocery bag to her car? – NoNo

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Formula for work

Work = Force x Distance• Force=Newtons (N)

• Distance=meters (m)

• Work=Newton-meters (N*m)– 1 N*m=1 Joule (J)

• Work=Joules (J)

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Work=Force*Distance

• Calculate: If a man pushes a concrete block 10 meters with a force of 20 N, how much work has he done?

• 200 joules200 joules(W = 20N x 10m)(W = 20N x 10m)

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PowerPowerthe rate at which work is

done

• Power = Work/Time• Power=(force x distance)/time

• Units: – Work=Joules (J)=Newtons*meters

(N*m)– Power=Watts = Joules/second

Who Needs It??Who Needs It??

ENERGY:ENERGY: POTENTIAL POTENTIAL & & KINETICKINETIC• Potential energy

– Stored up energyor energy held in readiness

–PE=mgh• PE=Joules (J)• m=mass (kg)• g=free fall acceleration (9.8

m/s2)

• h=height (m)

• Kinetic energy– Energy of motion

–KE= ½ mv2

• KE=Joules (J)• m=mass (kg)• v=velocity (m/s)

ENERGY:ENERGY: The ability to do The ability to do work & cause changework & cause change

• KINETIC ENERGY

– Energy of motion

– EXAMPLES

• moving bicycle

• moving auto

• high jumper jumping

• runner

• POTENTIAL ENERGY

– Stored-up energy, or

energy held in readiness

– EXAMPLES:

• Elastic Potential Energy

– Like slingshot or bungee cord

• Gravitational Potential Energy

– Like diver on diving board or light bulb in off position

POTENTIAL ENERGYPOTENTIAL ENERGY

Most Most potentialpotential

Energy stored or held in readiness

What is the relationship between Kinetic & Potential Energy?

http://www.classzone.com/books/ml_science_share/vis_sim/mem05_pg69_potential/mem05_pg69_potential.html

THE TRADE OFF Between Potential & Kinetic Energy

KINETIC ENERGYKINETIC ENERGYEnergy of motion

TYPES OF ENERGY

• MECHANICAL– Car, plane, wagon

• THERMAL– Heat from moving molecules– Inside all matter; furnace

• ELECTROMAGNETIC– Visible light, microwaves,

radio waves

• CHEMICAL– Batteries, matches,

body cells

• ELECTRICAL– Lightning, power lines

• NUCLEAR– Nuclear power plants,

atomic bomb, sun & stars

FORCES & MOTIONFORCES & MOTION

•FORCE–A Push or a Pull

•MOTION–A Change of Positionor Location

FRICTION AS A FORCE

•Strength of friction depends on:

–Types of surfaces involved

–How hard surfaces push together

TYPES OF FRICTION

• SLIDING FRICTION– 2 solid surfaces– Strength depends on roughness of surfaces

• ROLLING FRICTION– One solid rolls over the other– Much easier to move object

• FLUID FRICTION– Solid moving through a liquid or gas– Less than sliding friction

GRAVITY AS A FORCE

• FREE FALL

– Object accelerates as it falls due to gravity

– Object accelerates at rate of 9.8 m/s 2

• Every second velocity increases by 9.8 meters per second

• @ 2 seconds velocity is 9.8m/s + 9.8m/s = 19.6 m/s

• @ 3 seconds velocity is 9.8m/s + 9.8m/s + 9.8m/s = 29.4 m/s

GRAVITY AS A FORCE

•AIR RESISTANCE– Type of fluid friction that opposes motion

•WEIGHT– Measures force of gravity on object– Weight = Mass X Acceleration due to Gravity

WHAT MAKES OBJECTS WHAT MAKES OBJECTS

STARTSTART MOVING . . .MOVING . . .

STOP MOVING . . .STOP MOVING . . .

SLOW DOWN . . .SLOW DOWN . . .

SPEED UP . . .SPEED UP . . .

or CHANGE DIRECTION? or CHANGE DIRECTION?

NEWTON’S FIRST LAWNEWTON’S FIRST LAW

•An object at rest remains at An object at rest remains at rest and an object in motion rest and an object in motion remains in motion unless remains in motion unless acted upon by an outside acted upon by an outside force.force.

NEWTON’S SECOND LAWNEWTON’S SECOND LAW

• THE FORCE EXERTED BY AN OBJECT IS THE FORCE EXERTED BY AN OBJECT IS RELATED TO ITS MASS & ITS RELATED TO ITS MASS & ITS ACCELERATIONACCELERATION

• AN OBJECTS ACCELERATION IS RELATED AN OBJECTS ACCELERATION IS RELATED TO ITS MASS & THE FORCE REQUIRED TO TO ITS MASS & THE FORCE REQUIRED TO MOVE ITMOVE IT

• FORCE = MASS X ACCELERATIONFORCE = MASS X ACCELERATION

WHAT AFFECTS WHAT?WHAT AFFECTS WHAT?

FORCEFORCE ACCELERATIONACCELERATION

ACCELERATIONACCELERATION FORCEFORCE

MASSMASS FORCEFORCE

MASSMASS ACCELERATIONACCELERATION

FORCE = MASS X ACCELERATIONFORCE = MASS X ACCELERATION

NEWTON’SNEWTON’S THIRD LAW THIRD LAW

•For every action, thereFor every action, there

is an equal &is an equal &opposite reactionopposite reaction

THE END