#### Transcript File - 8th Grade Physical Science

Work, Power & Simple Machines What is “WORK?” Work is defined as A force acting through a distance In order for “work” to be done There must be a force (push, pull, lift, throw…) There must be movement over some distance The motion must be in the same direction as the force applied Work or Not Work Example Pushing a box across the floor A waiter carrying a tray of food to a table Pushing down on the pedal of a go cart Pushing on a locked door Direction of Force Direction of Motion Doing work? Yes Work or Not Work, Continued Example Pulling a suitcase on wheels through the airport Pushing a car down an incline Writing an essay in cursive Writing an essay on the computer Direction of Force Direction of Motion Doing work? Is work done? Your friend pushes his book to you by sliding it across the desk You lift the book off of the desk You hold the book in your hands You pass the book to another classmate You walk the book to another classmate Measuring Work Work is determined by considering Force and Distance Work is equal to the amount of force applied times the distance the object moves Work = force (newton) x distance (meter) W = F x D The unit for Work is the “newton-meter (N-m)” or the “joule(J)” If a force of 1 newton is applied to move an object 1 meter, then 1 newton-meter, or 1 Joule of work is done In this diagram the same amount of work is done using two different methods: A small force over a large distance A large force over a small distance Solving work problems If I push an object with a force of 50 Newton's, and it moves two meters as a result of that push, how much work is done? W=FxD W = 50 N x 2 m W = 100 N-m or 100 J Copy and complete….. If I lift an object 0.5 meters and the object has a weight of 150 N, how much work is done? If I carry the same object across the room 5 meters, how much work is done? What is “Power?” Power tells the rate, or “how fast,” work is being done; Power measures work in relation to time; Power is equal to the amount of work divided by the amount of time over which the work is done Calculating Power Power = work/time P = w / t P = (force x distance) / time The unit for work is the Joule The unit for time is the second The unit for Power is joules/sec (J/s) or the Watt (W) 1 Watt of power means that 1 joule of work is being done in 1 second P=w/t If more work is done in the same amount of time, what happens to the Power? It increases If the same amount of work is done in less time, what happens to Power? It increases Common Power Terms A 75 watt light bulb does 75 Joules of work in 1 second; it takes 75 watts of power to make the light bulb glow A kilowatt is 1000 watts How many Kilowatts of power are used by a 1200 Watt hair dryer? Solving Power Problems How much power is used when a person does 500 Joules of work in 20 seconds? P=w/t P = 500 / 20 P = 25 Joules/sec or 25 Watts Solving Power Problems A person weighing 600 N gets on an elevator. The elevator lifts the person 6m in 10 seconds. How much power was used to lift the person? P=w/t P = (f x d) / t P = (600 x 6) / 10 P = 3600 / 10 P = 360 J/s or 360 Watts