WORK, POWER, AND SIMPLE MACHINES

DO NOWS Write a list of 10 examples your idea of

work. What do all of these have in common? What do they require? How do you assess if work is done?

What is work? What is the formula? What are the SI units for work and what

do they represent?

WHAT IS WORK? Work is done only when a force moves

an object A force acting on an object and causing

it to move a distance is work Not every force is work..if you push

against the wall it does not move..that is not work!

Work = force X distance Work is measured in Joules

What is a joule exactly? (derive it)

If you pick up a bag of groceries and walk across the room the work is picking up the groceries not the walking.

The object must move some distance as a result of your force

The force you exert must be in the same direction as the objects motion.

Ie: the groceries

You walk

5

WHAT’S WORK? A scientist delivers a speech to an

audience of his peers. A body builder lifts 350 pounds

above his head. A mother carries her baby from

room to room. A father pushes a baby in a carriage. A woman carries a 20 kg grocery

bag to her car?

6

WHAT’S WORK? A scientist delivers a speech to an

audience of his peers. No A body builder lifts 350 pounds

above his head. Yes (Already in her arms) A mother

carries her baby from room to room. No

A father pushes a baby in a carriage. Yes

A woman carries a 20 kg grocery bag to her car? No

HOW CAN WE CALCULATE WORK?? Work can be determined by calculating Force used x distance moved = amount

of work

Therefore what is the formula for work? 

CALCULATIONS Work = force x distance  Joule – is the SI unit for work. Newton = force Meters = distance   Therefore if you exert: 1 Newton of force for 1 meter of

distance = 1 joule of work or 1N/m

Work is done when a force is exerted through a distance.

A student lifts a bag of books that weighs 135 N. If the bag is lifted .75 m, how much work does the student do?

F = 135 N d = .75 mW = FdW = (135 N)( .75 m)W = 101.25 J

COMPLETE PRACTICE PROBLEMS IN BOOK PAGE 285 (1-5)

COMPLETE MORE PRACTICE PROBLEMS HANDOUT

POWER Power tells you how fast something is

happening..how fast the work is being done

Power = work/time or Power = Force X Distance

Time Power is measured in watts (W) One watt is equal to 1 joule per second of

work divide joules/seconds

Power – the rate at which energy is transferred.

  P = W P = power

Watts t W = work

Joules t = time seconds   1 Watt (W) = 1 J/s  

HOW MUCH POWER MUST A MOTOR HAVE TO OPERATE A PUMP THAT RAISES 1500 KG OF WATER EVERY MINUTE A DISTANCE OF 12 M?

 

1. WDIK- m = 1500 kg t = 60 sd = 12 m

 2. Equations- P = W/t W = Fd F = mg  3. Plug and chug- F = mg = (1500 kg)(9.8 m/s2) =

14,700 N   W = Fd = (14,700 N)(12 m) = 1.76

x 105 J

P = W/t = (1.76 x 105 J)/(60 s) = 2940 W

 

http://www.glenbrook.k12.il.us/gbssci/phys/Class/energy/u5l1e.html

http://www.physicsclassroom.com

http://www.glenbrook.k12.il.us/gbssci/phys/Class/energy/u5l1e.html

www.Visualthesaurus.com

MECHANICAL ADVANTAGE - Demo: Use a ramp and 4 books and a spring scale and measure distance to move the 200g mass up vertically and horizontally on a rampCreate a data table use books as height w/ 200g hanging mass

1st Write a hypothesis –more –less- the same-work 2nd calculate the work for 1. vertically-straight up

2. up the ramp

MECHANICAL ADVANTAGE Mechanical Advantage – when you

increase distance you decrease force but the work remains the same.

 Machines – Multiply force redirect force- ie: pull down rope –lifts sail

work equation-  force x distance = work

Machines do not increase the amount of work. They spread out the distance so you don’t have to use the same amount of force to receive the same amount of work.

 Prove it: Work 32 J = work 32

J Force x distance force x

distance 8N x 4 m 4 N x 8 m

Ideal Mechanical advantage = ratio between output force and input force or output distance and input distance without friction

If you have force information use:  Output force /Input force = MA

If you have distance information use: Input distance/output distance =

MA

Mechanical advantage – multiplying force if you need 3200 N to lift a piano then use a ramp to exert 1600 N of force.

 

OF 3200N = 2 the ramp doubled your

IF 1600N force. Your output force is 2x your input force.

MA- is 2 no units

Mechanical Advantage – multiplying distance -you use a ramp that is 6 meters long to raise a piano 3 meters

 

ID- 6 meters = 2 the ramp doubled

OD 3 meters the distance

mechanical advantage of two

Write a paragraph on what you now know and did it differ from what you knew before,

Mechanical advantage to machines problem set /answers

http://library.thinkquest.org/CR0210120/Mechanical%20Advantage.html

MACHINES An instrument that makes work easier

is called a machine Machines do not have to be complex

electrical or gas powered deviced. Even simple objects can be a machine.

A pair of pliers would make it easier to take out a bolt so the pliers would be a machine

MACHINES CONT. There are two types of work involved in

using a machines: Work that goes into the machine (input) Work done by the machine (output)Work that comes out of the machine is

NEVER greater than the force that is applied to the machine or work that goes into the machine

MACHINES CONT. Machines make work easier because they

change either the size or the direction of the force put into the machine.

Machines multiply either the force or distance to make work easier, but never both!

The comparison of the work output to the work input is called efficiency.

The closer the amount of output is to the amount of input the more efficient the machine is.

EFFICIENCY CONT.

Efficiency is measured in percent and is never more than 100%. This is because the output can never be more than the input

The lower the friction of the machine the more efficient it will be. Keeping a car engine oiled makes it work better and more efficient

Efficiency – a measure of how much work that is put into a machine is changed to useful work; answer will be a percentage.

  efficiency = Wout x 100% Win Win = work put into the machine Wout = work put out by the

machine

Wout = Frdr Fr = resistance/output force

dr = resistance/output distance

Win = Fede Fe = effort/input force de = effort/input distance    For an ideal machine: Win = Wout Fede = Frdr

A worker applies an effort force of 20 N to pry open a window with a resistance force of 500 N. Find the mechanical advantage of the crowbar.

 

Fe = 20 N Fr = 500 N MA = ?

  MA = Fr = 500 N Fe 20 N MA = 25  

Find the effort force needed to lift a 2000 N rock, using a jack with a mechanical advantage of 10.

Fr = 2000 N MA = 10 Fe = ?

  MA = Fr / Fe

Fe = Fr / MA Fe = (2000 N)/(10) Fe = 200 N

SIMPLE AND COMPOUND MACHINES There are six types of simple machines:

Inclined plane Wedge Screw Lever Pulley Wheel and axle

INCLINED PLANE, WEDGE, SCREW A ramp is an example of an inclined

plane Simply put in inclined plane is a flat

slanted surface A wedge is an inclined plane that

moves and is usually made up of 2 inclined planes

The screw is an inclined plane wrapped around a center bar

LEVER AND PULLEY A lever is a rigid bar that pivots or moves

around a fixed point. A seesaw is an example

Fulcrum is the fixed point of a lever A pulley is a rope, belt or chain wrapped

around a grooved wheel A pulley can change the direction of a

force or the amount of a force When you use a pulley you change the

direction of the force you are applying.

WHEEL AND AXLE A wheel and axle is a simple machine

made up of two circular objects of different sizes

The wheel is the larger object the axle is the smaller one

Bicycle is an example of a wheel and axle.. The bike wheel is the large while and the sprocket the chain wraps around is the axle

SOURCES USED www.phs.d211.org/Science/okeefenm/

Okeefe/Okeefe/PhySci233/EnergyMachines/Mechanical%20Advantage.ppt –

www.cwcboe.org/gcms/teachers/apanagiotakis/Notes/Work%20&%20Power/Mechanical%20Advantage%20and%20Efficiency.ppt - Similar pages

education.jlab.org/jsat/powerpoint/0708_simple_machines_8.ppt -