# Work and Machines

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```					Work and Machines

Chapter 8
The Scientific Meaning of Work
   Definition: work                  effort might seem like
occurs when a force               a lot of work, but you
causes an object to               are not using a force to
move in the                       move an object.
direction of the
force.
   Use the definition
above, is doing
homework, work??
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:
   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
   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
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).

Nerdy Science Joke Break…

the criminals who
any work?

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…
   Calculate Power:
   What is the power
of a small motor
that can do 500 J
of work in 25
seconds?

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                   b. time/Work
c. Force x distance            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

W = 450 N x 1 m = 450 J   W = 150 N x 3 m = 450 J

   Mechanical
how many times the
machine multiplies
force.
   MA = output force
input force

   You apply 200 N to a machine, and the
machine applies 2,000 N to an object.
   Remember MA = output force/input force

Answer: MA = 2000 N = 10
200 N

Mechanical Efficiency
   Is a comparison of a machine’s work output
with the work input.
   Mechanical Efficiency = work output X 100
work input
Simple Machines
Levers
   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.

First Class Levers
   The fulcrum is between
the input force and the
Second Class Levers
the fulcrum and the
input force.
Third Class Levers
   The input force is
between the fulcrum
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.

Wedges
   A wedge is a double
inclined plane that
moves.
   Wedges are often
used to cut materials.
   They allow you to
an increased distance.
Screws
   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.
left screw are closer
together and wrap
more times around,
so that screw has a
greater mechanical
one on the right.
Wheel and Axle
   A simple machine
consisting of two
circular objects of
different sizes.
   Examples include a
door knob, a crank, a
fishing reel…

Pulleys
   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.
   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