Using machinesUsing machines

Compound machine- a Compound machine- a machine made up of 2 or machine made up of 2 or more simple machines.more simple machines.

Ex: an axe = wedge and leverEx: an axe = wedge and lever

What simple machines make up What simple machines make up these compound machines?these compound machines?

Efficiency- a measure of Efficiency- a measure of how much work put into a how much work put into a machine is changed to machine is changed to useful work.useful work.

Efficiency = W out / W in X 100Efficiency = W out / W in X 100

*Efficiency will be a percentage**Efficiency will be a percentage*

Efficiency Problem # 1Efficiency Problem # 1

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

Efficiency Problem # 2Efficiency Problem # 2

A 600 N box is pushed up a A 600 N box is pushed up a ramp that is 2 m high and 5 m ramp that is 2 m high and 5 m long. The person pushing the long. The person pushing the box exerts a force of 300 N. box exerts a force of 300 N. What is the efficiency of the What is the efficiency of the ramp?ramp?

Efficiency Problem # 3Efficiency Problem # 3

Using a ramp 6 m long, workers Using a ramp 6 m long, workers apply a force of 1,250 N to move apply a force of 1,250 N to move the 2,000 N crate onto a the 2,000 N crate onto a platform 2 m high. What is the platform 2 m high. What is the efficiency of the ramp??efficiency of the ramp??

Power- rate at which work is Power- rate at which work is done. Measure of amount of done. Measure of amount of work done in a certain work done in a certain amount of time.amount of time.

Power EquationPower Equation

VariableVariable Ending UnitsEnding Units

Power = PPower = P Watts (W)Watts (W)

Work = WWork = W Joules (J)Joules (J)

Time = tTime = t seconds (s)seconds (s)

W

P t

.

.

.

.

x

W = P x t

P = W / t

t = W / P

Power ProblemsPower Problems

1.1. If it takes 50 J of work to lift a If it takes 50 J of work to lift a box, how much power is box, how much power is needed to lift it in 2 seconds?needed to lift it in 2 seconds?

2. If a runner’s power is 130 W 2. If a runner’s power is 130 W as she runs, how much work is as she runs, how much work is done by the runner in 10 done by the runner in 10 seconds?seconds?

3. The power produced by an 3. The power produced by an electric motor is 500 W. How electric motor is 500 W. How long will it take the motor to do long will it take the motor to do 10,000 J of work?10,000 J of work?