Work, Power, and Machines

What would life be like without machines?

How would you get a heavy object up a hill without a machine?

Objectives Define work and power. Calculate the work done on an object and

the rate at which work is done. Calculate power. Use the concept of mechanical advantage

to explain how machines make doing work easier.

Calculate the mechanical advantage and efficiency of various machines.

Work & Power Work = product of

force and the distance over which it is applied.

Work = force x distance

W = Fd Units: newton meter

or joule n-m or j

Power = rate of doing work

Power = Work/time P= W/t Units: nm/s or j/s or

Watt746 w = 1 horsepower(see page 416 in your

book)So 3800w 1hp = 5 hp 746w

W

F d W

P tAdd these two formulas to your circle sheet!

Work problem sample A crane uses an average force of 5200 N

to lift a girder 25 m. How much work does the crane do on the girder?

W = Fd = 5200N (25m) W = 130000 or 1.3 x 105 J

Power problem sample While rowing in a race, John does 396 0 J

or work on the oars in 6 0.0 s. What is his power output in watts?

P = Work / time = W / t = 39 6 0 J / 6 0.0 s = 66 . 0 Watts, or w

Your assignment Is on page 415 /practice 1-3 And, as usual, SHOW:

The Formula Your work with units Your answer with units Because … It makes Mrs. C. happy

Answers to 415/1-3 1. W = Fd = 200N(1.5m) = 300 J P = W/T = 300J / 1.0 s = 300 w 2. W = Fd =15.0N(1.0m) = 15 J P = W/T = 15 J / 2.0 s = 7.5 w 3. W = Fd = 10.0N)(0.5m) = 5 J P = W/T = 5 J / 1.0 s = 5 w

Hydraulic crane http://science.howstuffworks.com/hydraulic-crane.htm

Machines and Mechanical Advantage Mechanical Advantage measures how

much a machine multiplies force or distance (Fout / Fin)

A machine can Make work easier Redistribute work Change size or direction of input force Increase output force by changing distance

over which force is applied

Inclined Plane and Mechanical Advantage Lift up a box Push a box up an

inclined plane

1 m1 m

Which is easier work? Lifting a heavy box 1 meter or pushing the box up an inclined plane?

Lifting the box 1 m into the truck W = Fd F = weight of the box = 225N W = 225N(1.00m) W = 225N-m or 225 J

Sliding the box up the inclined planeLet’s say the force to slide it up is only 75.0NThe distance you push the box up the

inclined plane is 3.00 m. W = F x d W = 75.0N x 3.00M W = 225 N-m or 225 j

What do you notice about the work being done? Lifting work = 225 J Pushing work = 225 J But… It takes (less or more) force? Less!!!

This brings us to MECHANICAL ADVANTAGE!!!M. A. = output force = input distance input force output distance (Actual) (Ideal)AMA compares forcesIMA compares distancesAMA is less than IMA.

For a machine to be helpful, its mechanical advantage must be greater than 1. MA>1

Turn to page 425 in your book and let’s look at the sample problem.

If a mechanic drives a car 1.8 m along a ramp to raise a car 0.3 m, what is the ideal mechanical advantage (IMA) of the ramp?

IMA = Distance in Distance out

= 1.8 m

0.3 m

IMA = 6

Your assignment is 425 / Practice 1-3

Answers to 425/Practice 1-31. IMA = input distance = 3 m = 6 output distance 0.5 m2. IMA = input distance = 0.5 m = 10 output distance 0.05 m3. Input distance = IMA(Output distance) =

2.5 (1.0m) = 2.5 m

Efficiency = Wout / Win x 100% Friction always works against machines

and makes a machine less efficient. Reducing friction increases efficiency.

Sample: If the efficiency of a machine is 75% and the machine requires 10.0 J of work input, find the work output.

Wout = (Eff x Win) = 75% (10.0 J) 100% 100% Wout = 7.5 J

Remaining assignments for Work, Power, and MachinesOn page 426 do math practice 8. and 9.On page 441 do reviewing content 1.

through 7.

Put these formulae on your circle sheet:

IMA = Din AMA = Fout Dout FinEff = Wout x 100% Win

Watts and horsepower 746 watt = 1 horsepower

A 750 W motor could be rated as ___HP.

750W 1 HP = 1.0 HP 746 W