WORK

TRANSFER OF

ENERGY

W = F W = F •• d d

When a force

causes a

displacement

CLIP

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Unit= JouleUnit= Joule

James Prescott JouleJames Prescott Joule (December 24, 1818–October 11, 1889) was an English physicist. •Educated at home•sent to Cambridge at the age of 16to study with the British chemist John Dalton•1850-established the equivalence between amounts of heat and mechanical work with his famous "paddle-wheel experiment". •Also established the cooling effect that occurs when a gas is rapidly expanded, an effect that is used in refrigeration systems.

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•Shared in discovering the law of the conservation of energy.•1840- stated a law, now called Joule's Law, that heat is produced in an electrical conductor. •The International unit of energy, the joule, is named in his honor.

Among his many inventions are "arc" or electrical welding and the displacement pump.

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For there to be work on an object, there

must be a displacement and the force must cause that

displacement

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WORK

Is work done here?1.pushing on a wall

2. pushing a boat out to sea

3. lifting up a box4. holding up a box

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When doing a chin-up, a physics student lifts her 40-kg body a distance of 0.25 meters in 2 seconds. How much work is done by the student's biceps?

W = F • d = (400 N) • (0.25 m) W = 100 J

Calculating Work 88

Work has nothing to do Work has nothing to do with the with the amount of amount of timetime that this force that this force acts to cause the acts to cause the

displacement.displacement.Sometimes, the work is done very Sometimes, the work is done very quickly and other times the work is quickly and other times the work is

done rather slowly.done rather slowly.

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POWER•The rate of energy transfer.

•How fast work is done.•Machines with different power ratings do the same amount of work...just in different amounts times.

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Son of a merchant, born in Greenock, Scotland, in 1736

Watt discovered how he could make the steam engine more efficient by cooling the used steam in a condenser that was separate from the main cylinder. This made them faster, safer, and more economical, resulting in steam overtaking water-power as the main source of energy.

Watt calculated that a horse exerted a pull of 180 lb., therefore, when he made a machine, he described its power in relation to a horse, i.e. "a 20 horse-power engine".

-1757:established his own mathematical instrument-making business

Unit of power is the Watt

James Watt

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One watt is equal to a power rate of one joule of

work per second of time.

watt (W)

1W = 1 J/s

1Kilowatt = 1000 Watts

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Calculating Power

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When doing a chin-up, a physics student lifts her 40-kg body a distance of 0.25 meters in 2 seconds. What is the power delivered by the student's biceps?

The work done to lift her body is

W = F • d = (400 N) • (0.25 m) W = 100 N-m W = 100 JThe power is the work/time ratio which is (100 J) / (2 seconds) = 50 Watts.

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TorquTorquee

•A torque is a force exerted at a distance from the axis of rotation; the easiest way to think of torque is to consider a door. When you open a door, where do you push? If you exert a force at the hinge, the door will not move; the easiest way to open a door is to exert a force on the side of the door opposite the hinge, and to push or pull with a force perpendicular to the door

•A measure of how much a force acting on an object causes that object to rotate.

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TorquTorquee

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