8th Grade Science

*Work and Power

*What is work?

*Examples anyone?

*Consider this: Before coming to school this morning you probably put your books in your backpack. You lifted the books and carried them with you. If you had a lot of books this may have seemed like a lot of work but; in the scientific definition of work, after you lifted the backpack you didn’t do any work at all.

*The Scientific Meaning of Work

*Work-force exerted on an object that causes it to move in the same direction in which the force is exerted

*If there is no motion, no work has been done no matter how much force is exerted

*The force exerted must be in the same direction as the object’s motion

Work. Not Work.

*Calculating Work

*Work= Force x Distance

10 N

40 m

W= FxD =10N x 40m =400 NxmThe SI unit for work is Newton x meter, which is known as a joule (J)

*Power

*Power- The rate at which work is done

*Power= Work/Time

*SI Unit: Joules/Second, known as Watts (W)

*Example: A tow truck exerts a force of 11,000N to pull a car out of a ditch. It moves the car a distance of 5m in 25 seconds. What is the power of the tow truck?

*power=(11, 000 N x 5.0 m)/25s

*=55, 000 J/25 s

*=2, 200 Watts

*Machines

*Machine- a device that allows you to do work in a way that is easier

*A machine makes work easier by changing at least one of the following:

*The amount of force you exert

*The distance over which you exert force

*The direction in which the force is exerted

*Machines provide a mechanical advantage

*Mechanical advantage= Input force/Output force

*Efficiency = (input force/output force) x 100