Transcript Using Machines PowerPoint

Chapter 5: Work and Machines
 Pick one of the
objects to the
right and describe
a situation in
which you would
use that object.
 Explain how machines make doing
work easier.
 Calculate the mechanical advantage of
a machine.
 Calculate the efficiency of a machine.
 Machine: a device that makes doing
work easier
 A machine does not change the amount
of work that is done!!!
 How does a machine make work easier?
1. They can increase the applied force.
2. They can change the direction of the applied
force.
3. They can increase the distance over which a
force is applied.
 Example: Car Jack
 The upward force exerted by the jack is
greater than the downward force on the
handle.
 The distance that you push the handle is
further than the distance the car is pushed up.
 Remember that W=Fd
 The work done by you (less force, more
distance) = work done by the jack (more
force, less distance)
 The jack increases the applied force, but
not the amount of work done.
 Example: A mover pushes heavy
furniture up a ramp instead of lifting it
directly onto the truck.
 W=Fd
 The work stays the same, however the
furniture moved a longer distance so less
force was needed to do the work.

Example: An ax splitting
wood
 You swing the ax
downward. The blade
changes the downward
force into a horizontal force
that splits the wood apart.
 There are two forces involved when a
machine is used to do work:
 Input Force (Fin): force applied to the
machine
 Output Force (Fout): force applied by the
machine

Example:
 When you try to pull a nail
out with a hammer, you
apply the input force on the
handle. The output force is
the force the claw applies to
the nail.

Two kinds of work need to be considered
when you use a machine:
 the work done by you on the machine (Win)
 the work done by the machine (Wout)

Example:
 When you use a crowbar , you
apply force to the handle and
make it move. The crowbar then
applies a force to another object
to make it move.
 Remember, energy is always
conserved!!!
 The amount of energy the machine transfers
to the object cannot be greater than the
amount of energy transferred to the machine.
 Wout is never greater than Win
 However, the machine does not
transfer all of the energy it receives!
 Some energy will be converted into heat
because of friction, so Wout is always
smaller than Win
 An ideal machine (no friction) would
have equal input and output.
 Win=Wout

Mechanical Advantage (MA): the number
of times a machine multiplies the effort
force.

.
 MA less than 1  input force is greater
than the output force.
 MA equal to 1  machine changes the
direction of the force.
 MA greater than 1  output force is
greater than the input force.

Calculate the MA of a crowbar to which you
apply 100 N of force to lift a 250 N rock.
 What does this mean?

Find the force needed to life a 2000 N
weight using a machine with a mechanical
advantage of 15.
 Efficiency: the measure of how much
the work put into a machine is changed
into useful output work by the
machine.
 If the machine has high efficiency, then
there is little heat created from friction.
 More Win is changed to useful Wout
 Efficiency =
Wout
Win
X 100
 Find the efficiency of a machine that
does 800 J of work if the input work is
2400 J.
Efficiency x Win
Wout =
100
Wout
Win =
Efficiency
x 100
 In an ideal machine, there is no friction
and the output work is equal to the
input work.
 The efficiency of an ideal machine is 100%
 The efficiency of a real machine is always
less than 100%
 Machines can be made
more efficient by reducing
friction.
 Oil or grease can fill the gaps
between the surfaces that are
in contact and allow them to
slide past each other more
easily.
 If you are building a house, would you
rather have tools that apply a large
mechanical advantage, or a small
mechanical advantage? Why?