A. What is work? Work and Simple Machines

A. What is work?
Work and Simple Machines
• Work is done on an object when the object
moves in the same direction that the force
is exerted.
• See examples p. 109
Chapter 4
Measuring work
Work = force x distance
W = Fd
Force is measured in
Newtons (kgm/s2)
Distance is measured in
meters (m)
Measuring power
Power = work done
time to do work
P=W
t
P=J
s
Power is measured in watts 1W = 1J/s
Units for problems
W = F x d
W = N xm
W = kgm/s2 x m
W = kgm2/s2 This is equal to a Joule
1J = 1N x m
or kgm2/s2
Work is measured in Joules
B. What is a machine?
• Machines make work easier by changing
one of at least 3 factors:
– Amount of force you exert
– The distance over which you exert your force
– The direction in which you exert your force
1
The Math Behind The Machine
work in = work out
In a perfect machine, these would be equal.
Things to know:
The force you put into the machine is the
effort force. (or input)
The force you must overcome is the
resistance force. (or output)
Efficiency
• Compares the output work to the input
work.
• Expressed as a percent
• In all machines some work is wasted due
to friction
Inclined Plane
• Less effort over a longer distance
• To find Mechanical Advantage:
length
height
Mechanical advantage
• Formula: actual mechanical advantage
Mechanical advantage = resistance force
effort force
MA = Fr or output
Fe
input
Simple machines make work easier by
giving you a mechanical advantage.
C. Types of Machines
• Inclined plane
• Wedge
• Screw
• Lever
• Wheel and axle
• Pulley
For each machine we will calculate the ideal
mechanical advantage
Types of Inclined Planes
1. Wedge – used to increase force, split
things apart or hold them together
Ex: teeth, knife, doorstop, nail, pin,
chisel (MA=length/width)
2. Screw – used to fasten, bore holes, lift
heavy weights or move air.
Ex: wall screw, drill bit, jackscrew,
propellers, fans
(MA=length threads/length screw)
2
Levers
• Things to know:
Load = what you are lifting or moving
Effort =where you apply the force to move the load
Fulcrum = the pivot point of a lever
MA of a lever
MA = distance from fulcrum to input (effort) force
distance from fulcrum to output (load) force
3 lever types
1st class – fulcrum is between the effort
(input) and the load (output)
nd
2 class – load is between the fulcrum and
the effort
3rd class – effort is between the fulcrum and
the load
(see examples in text p. 129)
Wheel and Axle
• 2 circular objects that rotate on a common
axis
• Larger object is the wheel, smaller one the
axle
• Increases force over a longer distance
MA = radius of wheel
radius of axle
Pulleys
3 Types:
1. Fixed pulley
2. Movable pulley
3. Block and tackle
1. Fixed pulley
• Only changes
the direction of
the force
• Can be
compared to
turning a first
class lever
(fulcrum in the
middle)
•The mechanical advantage is 1 –
you need as much effort as
resistance to move an object
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2. Movable pulley
• The pulley moves along a rope or wire
• Less force is used, but you need a greater
distance
• Can compare to a turning second class
lever – the resistance is in the middle
Mechanical advantage is 2 – the effort will always be
half the weight of the load
3. Block and Tackle
• Combination of fixed and movable pulleys
• Usually no more than 4 pulleys are used
(because of friction)
• This type of pulley gives a change in
direction and a gain in force
• To find the mechanical advantage, you
count the number of ropes
Archimedes Screw
Credits
Slide 10: http://www.uark.edu/depts/aeedhp/agscience/simpmach.htg/img6.gif
Slide 12: http://library.thinkquest.org/C004451F/movablepulley1.jpg
slide 14: http://library.thinkquest.org/C004451F/compoundpulley1.jpg
http://content.answers.com/main/content/img/McGrawHill/Encyclopedia/images/CE087300FG0010.gif
Arch screw: http://etc.usf.edu/clipart/27300/27350/archimedes_27350_lg.gif
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