Using Machines PowerPoint
Download
Report
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?