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# Chapter 5 Work and Machines

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```									Chapter 5 Work and Machines

Page 126-131
Review
• Motion
– Distance, time, speed (velocity), acceleration
– Speed = rate of change of distance
– Acceleration = rate of change of velocity
• Force
– Force, mass, acceleration
– A force is required to change the velocity of an object.
• F = ma
– Weight is the gravitational attraction on an object
• W = mg (g= 9.8 m/s2 the acceleration of gravity)
Review of Energy
• Energy is the ability to do work.
• Different forms of energy
• Different types of mechanical energy
– KE = ½ mv2, PE (GPE)= mgh
• Energy is always conserved
• Energy may be conserved but it is often changed
into a useless form (heat) by friction and air
resistance.
• But Einstein said, E = mc2
Work
• Work makes something move
• Work is the transfer of energy when a force makes
an object move.
• Two conditions
– A force must make something move
– The direction of motion must be in the direction of the
force
– Carrying books
• Work is a transfer of energy
• W= Fd (force in Newtons (kg m/s2, distance in m)
Practice Problems P 128
(turn in Monday)
• You push a refrigerator with a force of 100N.
If you move the refrigerator a distance of 5m
while you are pushing, how much work is
done?
• Given:
• Formula:
Power
•   Power is the rate of doing work.
•   How much work is done in 1 sec
•   P = W/t
•   W (joules) joules = kg m2/s2
•   Unit for power is joules/second called a Watt
•   A Watt is kg m2/s3
•   Since work is energy transferred
•   P= E/t
Power Example
• You push a box with a force of 100 N across
the floor 5 meters it takes you 45 seconds.
Your friend pushes a similar box also with a
100 N force also 5 meters across the floor but
it only takes your friend 30 seconds.
– How much work do you each do?

– How much power do you each expend?
Power Practice Problems P130
(Turn in Tuesday)
• You do 900 J of work pushing a sofa. If it took
5s to move the sofa, how much power did you
use?
– Given
– Formula
Review (P131)
•   Work is a transfer of energy
•   W= Fd
•   (force in Newtons (kg m/s2, distance in m)
•   N*m or Joules
•   Power is the rate of doing work.
•   How much work is done in 1 sec
•   P = W/t
•   W (joules) joules = kg m2/s2 , t= seconds
•   Joules/second = Watt
Machines (P132)
• A machine is a device that makes work easier.
• Simple machines (not motorized)
• How machines make work easier
– A machine can increase the force that can be
applied to an object.- Car jack (p 132)
– A machine can change the direction of the force-
splitting wedge (p133)
Work Done by Machines
• Figure 7 Page 133
• Choice – lift the chair straight up the height of
the truck.
• W = Fg x height
• Or- slide the chair up the ramp
• Work will be the same
• Distance will be greater but force will be less
Factors of Work
• Input force – the force applied to the machine
• Output force – the force applied by the
machine
• Input distance – the distance the input force
moves
• Output force – the distance the output force
moves.
• work in = input force x input distance
• work out = output force x output distance
Pulling a Nail
• work out = work in

•   Figure 10 page 135
•   hammer claw moves 1 cm to pull a nail
•   Handle moves 5 cm
•   Output force of 1,500 N
•   Input force = ?
Ideal Machines
• Workin= Workout

Forcein X distancein= ForceoutX distanceout

• Fin X 5cm = 1500N X 1 cm
• Fin= ?
• distance units cancel out if the same unit
Work, Distance and Force
Machines don’t decrease the work, but they can
decrease the force required.
Ideal machine (p135)
Workin= Workout

Wout= 100 N * 3m =        Win = Fin *5cm , Wout= Win
300J                      W= 300J but d=5m so F= 60N

5m
3m

100 N
4m
Workin= Workout
Forcein X distancein= ForceoutX distanceout
Fin X din= FoutX dout

F out d in
=
Fin    dout

IMA = F out = d in
Fin      dout
Real Machines
• Work out< Workin
• Some of the input work is always
converted into heat by friction.
• Efficiency tells us how much of the
input work is converted into output
work.
Workout x 100% < 100%
efficiency =
Workin
Practice
• Page 137 – Applying math 5-6-7
• Wednesday –
– AM Testing
– PM Problems
• Thursday
– Field Trip
•   Friday – Lab
•   Monday – Practice Problems
•   Tuesday – Chapter review
•   Wednesday- Chapter 5 Test
Simple Machines
Section 3
Page 138-146
Types of Simple Machines
• Simple machine does work with only one
movement.
• Lever
• Pulley
• Wheel and Axle
• Inclined Plane

• Compound Machines
Lever
• A bar that is free to pivot (turn) about a fixed
Point
• The fixed point is called the fulcrum
• Input arm – distance from the input force to
the fulcrum
• Output arm – The distance from the output
force to the fulcrum.
• If output arm is shorter than the input arm
then the output force is greater than the input
force.
Types of Levers (P138-139)
First-class Lever
Input Force
Output Force        Output                  Input Distance
Distance

Second-class Lever      Output             Output Force
Distance

Input Distance
Input Force

Output
Third-class Lever                         Distance                    Output Force
Input Distance

Input Force
of a Lever
Force output
IMA =
Force input
Force output x Length output=Force input X Length input

L input
IMA =
L output
Pulley Types (P 141-142)
Fixed Pulley

Output Force 4N                Input Force 4N

4N

Movable Pulley                2N
Input Force 2N

Output Force 4N
4N
Types of Pulleys (cont)
Block and Tackle

1N
1N
1N
Input Force 1N
1N

Output Force 4N
4N
of a Pulley
Force output
IMA =
Force input

IMA =   Number of Stings Lifting the Load
Wheel and Axel (P143)

Input Force
rw   ra
Output Force

IMA =
Inclined Plane
W= 100 N * 3m = 300J        W= 300J but d=5m so F= 60N
F= 100N, height = 3m

5m
3m

100 N

Length of Slope
IMA =
Height of Slope
Types of Inclined Plane
•   Screw
•   Wedge
•   IMA = length of slope/height of slope
•   Page 144
Compound Machines
• Two or more simple machines
that operate together.
–Can opener
–Automobile
–Space shuttle
Machines
Force output > 1
All Simple Machines IMA =
Force input
L input
Lever    IMA =
L output
Inclined Plane              Length of Slope
IMA =
Height of Slope
Wheel and Axle         Radius of wheel
IMA =
Pulley System
IMA =    Number of Stings Lifting the Load
Section Review
• Page 146 Applying Math
• 5-6-7
Formulae

F = m*a , w= m*g    unit kg m/s2 = Newton

W= F*d units N*m = Joule

P= W/t   or   P= E/t       unit J/s = Watt

Workout x 100% < 100%
efficiency =
Workin
Assignment
•   Chapter 5 Review
•   Page 152-153
•   8,10,11,12,13,15,16,17,19,20,24-28
•   Due on Tuesday
•   Test on Chapter 5 Wednesday
– Work and Power
– Simple Machines
• IMA
• Efficiency

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