Work, Power and Efficiency

Work occurs when a force causes an object to move -the object will move in the direction of the force.

If you push a car , but the car does not move, has work been done If you push a car , but the car does not move, has work been done? Why or why not?

Doing work on an object transfers energy to the object. Moving objects have kinetic energy

Doing work on a tray transfers energy to the tray Doing work on a tray transfers energy to the tray. The added energy can be either kinetic energy or potential energy.

Check your understanding: Visual check page 331 Explain your answer

Calculating work Unit of measurement for work is the W = F x D Unit of measurement for work is the newton – meter , simply called the joule (J) A man applies a force of 500N to push a truck 100 m down the street. How much work does he do?

A student pushes a desk 2.0 m across the room using a constant force of 25N. How much work does the student do on the desk?

Lesson 1-2

Bell Ringer: In which situation do you do more work? You lift a 75 N bowling ball 1 m off the floor. Or You lift a 50 N bowling ball 2 m off the floor. Explain Check understanding: Visual Check p 332

What is power? Power is the rate at which work is done. You can also think of power as how fast energy is transferred to an object

How to calculate power P = work (J)_ time (s) Unit of measurement for power is joules / second or watts (W)

A boy does 18 J of work in 2.0s on his backpack as he lifts it from a table. How much power did he use on his backpack?

Lesson 1-4

How can you increase the power?

How much power does it take to do 500 J of work in 10 sec?   How much power does it take to lift 250 N, 40 m high in 2 seconds? If it takes 20 N to move a box, how much power will be needed to move the box a distance of 5 meters in 5 seconds?

Exit Ticket: Do you agree or disagree? Explain 1. Work is done when you push a book across a table. 2. Doing work faster requires more power.

What is efficiency? Machine – any device that makes doing work easier Input force – force you apply to a machine Output force – the force the machine applies

The work you do is called the work input, The work done by the machine is called the work output

Efficiency The output work done by a machine never exceeds the input work . Friction converts some of the input work to thermal energy. a machine’s efficiency is always less than 100%

What is the efficiency of a hammer if the output work is 100 J and the input work B. 40% C. 80%

Exit Ticket: A man expends 100J of work to move a box up an inclined plain. The amount of work produced is 80J. What is the man's efficiency? A. 20% B. 80% C. 800% D. 200% You pull a nail out of a piece of wood using a hammer. Explain why the nail feels warm.

Simple Machines A device with few, if any, moving parts that makes it easier to do work

Lever - consists of a bar and a pivot point

Wheel and axle

Pulley – is a wheel that turned by the action of a rope

Inclined plane – a straight, slanted surface

Screw – an inclined planed that is wrapped around a central bar

Wedge – inclined plane that changes the direction of applied force

Machines make work easier because they change the size or direction of the input force. Machines do not save work.