ENERGY

Energy is the ability to do work.

ENERGY• The units of energy are:

– Joule

– calorie (4.2 Joule)

– Kilocalorie• Also known as a food calorie• Often written as Calorie in the United States

FOOD LABELS IN UK

DOG FOOD LABEL US

BENEFULDOG FOOD

ACTIVITY

JOULE OF A COOKIE

Question

How many Joules of energy are stored in a candy bar which contains 250 food calories?

A. 250000 B. 1050 kJC. 840 kJD. 420000 J

ENERGY

The forms of energy are:

Heat

Light

Electrical

Chemical

Sound

Nuclear

Mechanical

ENERGY

Energy can be converted into work or from one form to another.

ENERGY

Conservation of Energy

In any process, energy can not be created or destroyed. It can only be converted from one form to another.

ENERGY• MECHANICAL ENERGY

– The sum of the potential energy and kinetic energy of an object

• POTENTIAL ENERGY– Energy of position

• KINETIC ENERGY– Energy of motion

ENERGY

POTENTIAL ENERGYnormally associated with the work required to lift an object or distort an elastic object

ENERGY

GRAVITATIONAL POTENTIAL ENERGY (GPE)– Is equal to the work required to lift an

object

GPE = Weight X Height

Video

GPE OF SAND MOVING A TOY

ENERGY

• Question– A 500 N girl stands on a diving platform 10

meters above the water. A 1000 N boy stands on the platform 5 meters above the water. Which possesses the most GPE?

– Answer – neither, the GPE is the same for both

ENERGY

ELASTIC POTENTIAL ENERGY (EPE)–Is equal to the work required to

bend or stretch an object

Video

EPE IN A POLE VAULT

ENERGY

KINETIC ENERGY (KE)–The kinetic energy of an object

depends on its mass and velocity (speed).

KE = ½ (mass) (velocity)2

ENERGY

HOW TO FIND THE INCREASE IN KINETIC ENERGY OF AN OBJECT WHEN ITS SPEED CHANGES

KE1/KE2 = ½ m V12 / ½ m V2

2

KE1/KE2 = V12 / V2

2

KE1/KE2 = (V1 / V2)2

ENERGY

TO FIND BY WHAT FACTOR THE KE CHANGES, FIND THE FACTOR FOR THE SPEED AND SQUARE IT

ENERGYQuestion

–How much more kinetic energy does a car have at 60 mph as compared to 30 mph?

–Answer: four times as much kinetic energy

ENERGY

• Conservation of energy in mechanical systems

• The energy you start with equals the energy you finished with plus any work done.

• KEf + PEf = KEs + PEs + Work

ENERGY

• Conservation of energy in a pendulum

• PhET Simulation “Pendulum Lab”

ACTIVITY

KINETIC PENDULUM

POTENTIAL POPPER

ENERGY

• Conservation of energy in a mass on a spring

• PhET Simulation “Mass – Spring Lab”

ENERGY

• Conservation of energy on a track or roller coaster

• PhET Simulation “Energy – Skate Park””

ACTIVITY

MECHANICAL ENERGY USING PhET

VIDEO

ROLLER COASTER

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QUESTIONA pendulum is pulled to point 1. It is released from rest and allowed to swing back and forth. If the pendulum possesses 100 joules of energy, which point listed below is assigned the wrong value for the PE or KE?

A. Point 5 - KE = 100 J

B. Point 4 - PE = 50 J

C. Point 3 - KE = 100 J

D. Point 2 - KE = 50 J

E. Point 1 - PE = 100 J

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POWER

Power is the rate of doing work.

Video

Human Power

POWER

• Power can be found by dividing the amount of work (W) done by the amount of time (t) required to do the work.

P = W / t

ACTIVITY

POWER

• The unit of power is the watt in ISU.– Watt = Joule / second

• The unit of power in the British system is the horsepower.– Horsepower = 750 Watt– 550 foot pound per second

POWER• Questions

– Which can do more work, a 100 kw motor or a 200 kW motor?

– If a 100 kW motor can lift an elevator from the first to the fourth floor in 10 seconds, how long will it take a 200 kw motor to lift the same elevator the same distance?

SUMMARY

• FORCE• WORK • ENERGY

– FORMS– CONSERVATION OF ENERGY– MECHANICAL SYSTEMS

• POWER