2.10 understanding work, energy, power and efficiency

7/27/2019 2.10 Understanding Work, Energy, Power and Efficiency

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Learning Outcome :

Define work ( W ) as the product ofan applied force ( F ) anddisplacement ( s ) of an object in thedirection of the applied force i.e. W =Fs

State that when work is done energy

is transferred from one object toanother.


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What is Work?

Work done by a constant force isgiven by the product of the force andthe distance moved in the direction ofthe force.

Unit: Nm or Joule (J)

Work is a scalar quantity.


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Formula of work


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Example 1 :

A student is pulling an object and the force he applied is 7 N at

an angle of 60 to the horizontal. If the object moves a

horizontal distance of 0.3m, what is the work done by the force. Hint: the force 7 N can be resolved into a horizontal component

7 cos 60 and a vertical component 7 sin 60


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When the displacement is in thedirection of force

Formula of work 2

When the direction of force and motion are

same 0, therefore cos 0 = 1

Work done,

W = F x s x cos 0

= F x s


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Example 1

A student is pulling an object on a horizontaltable.

If the spring balance reads 5N and the objectmoves a distance of 0.6 m, what is the workdone by the student?


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Example 2 :

Diagram above shows a 10N force is pulling a metal.

The friction between the block and the floor is 5N. If the

distance travelled by the metal block is 2m, find

the work done by the pulling force

the work done by the frictional force


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Asnwer:

(a) The force is in the same direction of the motion.

Work done by the pulling force,

W = F s = (10)(2) = 20J (b) The force is not in the same direction of motion,

work done by the frictional force

W = F x s cos180= (5)(2)(-1) = -10J


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Transfer of Energy When Workis Done

Machine that do work must be supplied

with energy.

As the machine does work, energy istransferred from one object to another.

Energy transfers are never 100% efficient

since some energy is wasted when it is

converted to other form which are wanted.


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Striking Match-Stick

Work is done against friction when amatch stick is rubbed against therough side of a match stick box.

The energy to do work comes fromthe chemical energy stored in thefood we eat.

The chemical energy is changed toheat energy used to light up thematch stick


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Walking Up A Flight of Steps

The boy does work when he walks up aflight of steps.

The energy to do work comes from the

chemical energy of the food he eats. The chemical energy is being changed

to gravitational potential energy, whenthe boy is walking up the flight ofsteps.


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Work Done to Move ObjectUpward

Work Done Against the Force ofGravity


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Example :

Ranjit runs up a staircase of 35 steps. Eachsteps is 15cm in height. Given that Ranjit'smass is 45kg, find the work done by Ranjit to

reach the top of the staircase. Answer:

In this case, Ranjit does work to overcome thegravity.

Ranjit's mass = 45kgVertical height of the motion,h = 35x 0.15 mGravitational field strength, g = 10 ms-2Work done, W = ?


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Finding Work Done from aForce

Force - Displacement

Graph

In a Force-Displacementgraph, work done is equal to

the area in between the

graph and the horizontal

axis.


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Answer:

In a Force-Displacement graph, work

done is equal to the area below the

graph. Therefore, work done

W = x 8 x 10 = 40 J


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Kinetic Energy

Define kinetic energy and state that E= mv2

Define gravitational potential energy andstate that Ep = mgh.

State the principle of conseration of

energy.

Define power and state that P = W/t

Explain what efficiency of a device is.


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Will back

Force applied, F = ma = mv/ 2s

Work done = Fs = mv

Work done is changed to kineticenergy of the object.

KE = mv


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Example :

Determine the kinetic energy of a2000-kg bus that is moving with aspeed of 35.0 m/s.

Answer:

Kinetic Energy,

E = mv

= 1225000J


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To Show Gravitational PotentialEnergy = mgh


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Example : Ice Climber

A ice climber gaingravitational P.E as heclimbs higher.

He needs to wear spiked

shoes and has to carry anaxe to increase frictionalforce so that he wont slipdown.

It he slips down, his P.E.gained is converted to K.E.


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To Show the Principle ofConservation of Energy


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Answer:

In this case, kinetic energy isconverted into heat energy due tothe friction. The work done toovercome the friction is equal tothe amount of kinetic energyconverted into heat energy, hence


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Example of EfficiencyCalculation


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2.10 understanding work, energy, power and efficiency

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