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
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?
5
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
6
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 m
W = Fd
W = (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

t





1 Watt (W) = 1 J/s
P = power
W = work
t = time
Watts
Joules
seconds

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 s d = 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/Cl
ass/energy/u5l1e.html

http://www.physicsclassroom.com
http://www.glenbrook.k12.il.us/gbssci/phys/Cla
ss/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 ramp
Create 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

force
equation-
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
4Nx8m

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/Mec
hanical%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.



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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.




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efficiency = Wout x 100%
Win
Win = work put into the machine
Wout = work put out by the machine
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

Wout = 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
thecrowbar.

Fe = 20 N
Fr = 500 N





MA = Fr = 500 N
Fe 20 N
MA = 25
MA = ?

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





MA = Fr / Fe
Fe = Fr / MA
Fe = (2000 N)/(10)
Fe = 200 N
Fe = ?
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

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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/Mechani
cal%20Advantage.ppt –
 www.cwcboe.org/gcms/teachers/apanagiotaki
s/Notes/Work%20&%20Power/Mechanical%2
0Advantage%20and%20Efficiency.ppt - Similar
pages
 education.jlab.org/jsat/powerpoint/0708_simp
le_machines_8.ppt 