Work and power

Work and Power

Work

Work is the transfer of energy that occurs when a force makes an object move. 2 conditions for work to be done

An applied force must make the object moveMovement must be in the same direction as the

force.

Force

Movement

Work

How is the scientific definition of work different from the every day meaning?

For

ce

Mov

emen

t

Work

When work is done transfer of energy always occurs.

When you work on an object, you increase its energy.

Calculating Work

Work = applied force * distance Or w=Fd

Create a calculation triangle to make it easier to solve for “w”.

W

F x d

Calculating Work cont.

A force of 75N is exerted on a 45 kg couch and the couch is moved 5m. How much work is done in moving the couch?

The brakes on a car do 240,000 J of work in stopping the car. If the car travels a distance of 50m while the brakes are being applied, what is the total force of the brakes on the car?

Power

Power is the amount of work done in one second. Rate at which work is done

Calculating Power

Power = work/ timeCreate a calculation triangle to make it

easier to solve for “p”.

W

P x t

Calculating Power cont.

To lift a baby from a crib 50 J or work are done. How much power is needed to lift the baby in 0.5s?

The power produced by an electric motor is 500 W. How long will it take the motor to do 10,000 J of work?

Machines

A machine is a device that makes doing work easier. It increases the force that you apply.

Machines cont.

Input Force: force that is applied to the machine. = Fin

Output Force: the force applied by the machine. = Fout

Mechanical advantage is the ratio of the output force to the input forceMA = Fout/ Fin

Efficiency % = Output work/ input work x 100

Machines cont.

Friction causes energy of the output force to be lost as heat.This means Wout is smaller than Win

How would lubricating a machine affect the output force exerted by the machine?

Ideal MA is one without friction

MA and Efficiency Triangles

Create a triangle for

Mechanical

Advantage

Create a triangle for

efficiency.

Fout

MA x Fin

Fout

Eff. x Fin

x 100%

Calculating MA and Efficiency

The mechanical advantage of a hammer if the input force is 125 N and the output force is 2,000 N.

Find the efficiency of a machine that does 800 J of work if the input work is 2,400 J.

Simple Machines

6 basic typesLeversPulleysWheel and Axel Inclined PlanesScrewWedge

View Simple Machines

Levers

There are 3 types of Lever:First-Class: Fulcrum is between input &

output force.Second-Class: Output force is between the

fulcrum & the input forceThird-Class the input force is between the

fulcrum and the output force.

View Examples

Pulleys

Fixed: Changes only the direction of YOUR force. (4N in gets 4N out)

Movable: the attached side of the rope supports half of the weight. Ex. For a 4N object attached side supports 2N while you only do 2N of work.

Block and Tackle: Made up of 2 fixed & 2 movable pulleys & weight is divided evenly among all of them.

View Pulleys