8 th Grade Science

* 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.

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

* 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

* 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