C HAPTER 14.1 W ORK & P OWER Pgs

L EARNING O BJECTIVES Recognize when work is done. Calculate how much work is done. Explain the relationship between work and power.

N EW V OCABULARY Work : done when a force causes an object to move in the same direction that a force is applied Power: rate at which work is done

W HAT IS WORK ? In science work has a different meaning that just chores or jobs that must be completed Work : done when a force causes on object to move in the same direction that a force is applied In what ways have you done work today?

W ORK & M OTION In order for you to do work, two things must happen You must apply a force to an object The object must move in the same direction as your applied force You do work on an object only when the object moves as a result of the force you exert.

A PPLY F ORCE AND D OING W ORK To do work, an object must move in the direction a force is applied Example: Lifting a laundry basket (fig. 2, pg 427) Arms exert a force upward as you lift the basket; the basket moves in the direction of the force your arms applied If you carry the basket forward, do you arms still do the work? No, they do not. Your legs are now doing work to move your body & the basket forward No work is done when the motion of an object is at a right angle to the applied force

F ORCE IN T WO D IRECTIONS Sometimes only part of the force you exert moves an object Example: pushing a lawn mower (fig.3, pg 427) You push at an angle to the ground Part of the force is to the right and part is down Only part of the force that’s in the same direction of the mower (to the right) does the work

C ALCULATING W ORK Work is done when force makes an object move More work is done when the force is increased or the object is moved a greater distance Work is calculated using the following equation: Applying Math: pg 428 Practice problems 1 & 2 The SI unit of measure for work is Joules Work= joules Force = Newtons Distance = meters To calculate work, the distance is not the distance the object moved total, but the distance the object moves while the force is being applied.

W HAT IS P OWER ? Power is the rate at which work is done What does it mean to be powerful? If two weightlifters both lift the same amount of weight the same vertical distance, the both do the same amount of work. How much power do they use – the same or different? The weightlifter who lifted the fastest is more powerful

C ALCULATING P OWER Power can be calculated by dividing the amount of work done by the time needed to do the work. Power is measured in watts Applying Math, pg 429 Complete 1 & 2 Work is measured in Joules Time is measured in seconds

W ORK & E NERGY If you push a chair and make it move, you do work on the chair and change its energy When something is moving it has energy of motion (kinetic energy) You also change the energy of an object when you do work and lift it higher An object has potential energy that increases when it is higher above Earth’s surface You are increasing its potential energy

P OWER AND E NERGY When you do work on an object you increase the energy of the object Energy can never be created or destroyed If an object gains energy, it must lose energy When you do work on an object you transfer energy to the object and your energy decreases The amount of work done is the amount of energy transferred Power is also equal to the amount of energy transferred in a certain amount of time

A SSESS Y OUR L EARNING Describe a situation in which work is done on an object. Evaluate which of the following situations involves more power: 200 J of work done in 20 sec or 50 J of work done in 4 sec. Explain your answer Determine two ways power can be increased. Calculate how much power, in watts, is needed to cut a lawn in 50 min if the work involved is 100,000 J Applying Math : complete problems 7 pg 430