NIS – PHYSICAL SCIENCE

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Lecture 19Work

Ozgur Unal

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What is Work?Work is the energy transferred when a force makes an object move.A force must move an object to do work.Forces do work.Examples: Lifting up a box, pushing a desk forward

If a heavy object stays at rest as you push it, do you do work?

No, because the force you apply does not do any work.

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Work and EnergyCan you state the relationship between work and energy?When you do work on an object (such lifting up an object), do you change its energy?Energy is transferred when a work is done on an object.As you lift up an object, its GPE increases.As you push an object forward, you increase its kinetic energy. Therefore, you do work.

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Work and EnergyIn some cases, the work done on an object decreases the energy of that object.Example: Slowing down a fast moving object by applying an opposite force on it.Therefore, we say that “energy is transferred when work is done”.Energy is transferred from the object doing the work to the object on which work is done.

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Calculating WorkConsider this situation:You apply a force on an object at rest to move it for 20 meters.Does the amount of energy transferred from you to the object increase if you increase the force you apply?Does the amount of energy increase if you move it with the same force for a longer distance (i.e. for 40 meters)?If you apply a bigger force, the object will gain more speed (there fore kinetic energy).If you apply the force for a longer distance, the object will gain more kinetic energy.

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Calculating WorkWork done on an object is proportional to the force doing the work and the distance the object travels in the direction of the force.

Work = Force x distanceW = F x d

What is the unit of work?Is work a vector?

Example: A force of 75 N is exerted on a 45-kg couch, and the couch moved 5 m. How much work is done by the force in moving the couch?

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When is Work Done?When you calculate work done on an object, you need to consider two questions first.Does the object move a distance in the direction of the force applied?Does the force act on the object throughout the whole distance?

Example: Suppose you throw a ball with a force of 2 N and the ball moves 10 meters. Can you calculate the work done?No, in order to calculate work, you need to find the distance over which the force is applied. As the ball moves in the air, you do not apply any force on it. Once the ball leaves your hand, you do not do any work on it.

NIS – PHYSICAL SCIENCE

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Lecture 20Power

Ozgur Unal

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PowerWhat is power?How is it related to work?

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PowerWork is the energy transferred when a force moves an object.The rate of energy transfer might differ.Example: Pushing an object along a ramp slowly or fast.

Example: Let’s say, you do 10 J of work on an object by lifting it up in 10 seconds. And your friend lifts the same object in 20 seconds.The rates you do the same amount of work are different. Your rate of doing work is faster than your friend.

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PowerPower is the amount of work done in unit time.Power is the rate at which work is done.

Power = Work / Time P = W / t

Unit of power is Watt (W)

Example: Let’s say, you do 10 J of work on an object by lifting it up in 10 seconds. And your friend lifts the same object in 20 seconds.The rates you do the same amount of work are different. Your rate of doing work is faster than your friend.Pyou = W / 10 s = 10 J / 10 s = 1 W

Pfriend = W / 20 s = 10 J / 20 s = 0.5 W

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PowerExample: To lift a baby from a crib, 50 J of work is done. How much power is needed if the baby is lifted in 0.5 s?

Example: If a runner’s power is 130 W, how much work is done by the runner in 10 minutes?

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Power and EnergyRemember the definition of work.Power can also be expressed in terms of the energy transferred from one object to the other.

Power = Energy transferred / Time P = E / t

Example: LightbulbsThe filament in the lightbulb converts the electrical energy into heat and light.The power of the lightbulb is the amount of energy transferred to the lightbulb each second.