Transcript Work and Machines - DAWG SCIENCE

Work and Machines
Chapter 8
The Scientific Meaning of Work

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Definition: work
occurs when a force
causes an object to
move in the
direction of the
force.
Use the definition
above, is doing
homework, work??
Answer: No mental
effort might seem like
a lot of work, but you
are not using a force to
move an object.
http://www.youtube.com/watch?v=pbsSiFgW-Xw&feature=related
Working Hard, or Hardly Working?

Applying a force
doesn’t always result
in work being done.

For example, pushing a
car that won’t budge.
If you can’t get it to
move you haven’t done
any work!
Work continued

Force and motion in the
same direction:
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For work to be done, the
object must move in the
same direction as the
force.
For example if you are
carrying a heavy suitcase
through the airport it is
not considered work
because the direction of
force is upward, and the
direction of motion is
forward!
Work or Not Work?
Calculating Work
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Work = force x distance
W= F x d
If a man applies a force
of 500 N to push a truck
100 m down the street.
How much work does he
do?
W=Fxd
Answer:
W = 500 N x 100 m
W = 50,000 J
The unit used to
express work is the
newton-meter, but is
more commonly
called the joule (J).

http://www.youtube.com/watch?v=IvcOYOO0Fvw&feature=related
Nerdy Science Joke Break…

Did you hear about
the criminals who
never had to do
any work?
Answer: They were
joule thieves.
Power
Power is the rate at which work is done.
 It is how fast work happens!
P=W/t
 Power = Work
time
 The unit used to express power is joules
per second (J/s), which is more simply
called the watt (W).
 For example if you do 50 J of work in 5
seconds, your power is 10 J/s or 10 W.

Try it…

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Calculate Power:
What is the power
of a small motor
that can do 500 J
of work in 25
seconds?
http://www.youtube.com/watch?v=5EsMmdaYClQ
Nerdy Science Joke…
Q: “What is the unit of
power?”
A: “Watt.”
Q: “I said, what is the unit
of power?”
A: “WATT!”
Q: “I SAID…”
Quiz 8.1
1. In which situation do you do more work?
W=Fxd
a. You lift a 75 N bowling ball 2 m off the floor.
b. You lift two 50 N bowling balls 1 m off the floor.
2. The rate at which work is done is:
a. Joule
b. Time
c. Power
d. Work
3. Power is:
a. Work/time
c. Force x distance
b. time/Work
c. Work/Force
Quiz 8.1 continued…
4. The unit used for Power is:
a. newton-meter
b. Joule
c. meter per second
d. Watt
5. You push a 100 N box 3 m across the
floor. How much work has been done?
a. 33.3 J
b. 300 J
c. 33.3 W
c. 300 W
Machines


A machine is a device that helps make
work easier by changing the size or
direction of a force.
The work that you do on a machine is
called work input.


Called input force
The work done by the machine is called
work output.

Called output force
The force-distance trade off
W = 450 N x 1 m = 450 J
W = 150 N x 3 m = 450 J
Mechanical Advantage
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Mechanical
Advantage tells you
how many times the
machine multiplies
force.
MA = output force
input force
Finding Advantage
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You apply 200 N to a machine, and the
machine applies 2,000 N to an object.
What is the mechanical advantage?
Remember MA = output force/input force
Answer: MA = 2000 N = 10
200 N
http://www.youtube.com/watch?v=oWiZ_5qvs7I&feature=related
Mechanical Efficiency
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Is a comparison of a machine’s work output
with the work input.
Mechanical Efficiency = work output X 100
work input
Simple Machines
Levers

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A lever is a simple
machine consisting of
a bar that pivots at a
fixed point, called a
fulcrum.
There are 3 types of
levers.
http://www.youtube.com/watch?v=PW7ztbwJKBk&feature=related
http://www.youtube.com/watch?v=wV1pYkTtsxg&feature=related
First Class Levers
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The fulcrum is between
the input force and the
load
Second Class Levers

The load is between
the fulcrum and the
input force.
Third Class Levers

The input force is
between the fulcrum
and the load.
Inclined Planes


A simple machine
that is a straight,
slanted surface.
An inclined plane
allows you to apply a
smaller force over a
greater distance.
http://www.youtube.com/watch?v=pG_kT565-XQ&feature=related
Wedges
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A wedge is a double
inclined plane that
moves.
Wedges are often
used to cut materials.
They allow you to
exert your force over
an increased distance.
Screws
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An inclined plane that
is wrapped in a spiral.
When you turn a
screw, you exert a
small input force over
a large turning
distance, but the
screw itself doesn’t
move very far.
Mechanical Advantage of Screws

The threads on the
left screw are closer
together and wrap
more times around,
so that screw has a
greater mechanical
advantage that the
one on the right.
Wheel and Axle
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A simple machine
consisting of two
circular objects of
different sizes.
Examples include a
door knob, a crank, a
fishing reel…
http://www.youtube.com/watch?v=C-UXry7OiXM&feature=related
Pulleys
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A simple machine
consisting of a grooved
wheel that holds a
rope or a cable.
A load is attached to
one end of the rope,
and input force is
applied to the other
end.
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Fixed Pulleys only
change the direction
of the force.

Movable Pulleys are
attached to the object
that is being moved.
Compound Machines

Machines that are
made of two or
more simple
machines.