Download Science 8: Unit D: Mechanical Systems

Science 8: Unit D:
Mechanical Systems
Topic 1: Levers and Inclined
Planes
What is Work?
 The scientific definition of work is when a force is
done on an object causing that object to move in
the same direction as the force. If there is no
movement, then no work is being done, no
matter how much force is being used.
 Example1: When you jump straight up, your leg
muscles do work in lifting you up, as you move
back down to the ground, the Earth’s gravity is
doing work in pulling you back down.
Work Continued
 Example 2: A man runs out of gas 5 km away
from the gas station. He tries to push the car the
rest of the way, but cannot move it. No work is
being done even though he’s pushing with force.
 The amount of work is calculated by multiplying
the force times the distance the object moves.
The formula looks like this: W = F x d
 Force is measured in Newtons (N) and distance
is measured in meters (m). The resulting work
unit is called a joule (J).
Energy and Work
Work is done when there is a transfer of
energy and movement occurs. Energy
provides the force needed to make an
object move. A machine transfers
energy from its source to the object,
causing the object to move. A bicycle
transfers the energy from your moving
legs to the energy of moving bicycle tires.
Work and Machines
 The job of MOST machines is to decrease the
amount of force you have to use to perform a
specific task. A car jack allows you to lift up a car
without you exerting a large amount of force to
do it. The amount of work to do a job never
changes whether you use a machine or not. So
if force decreases and work stays the same that
would mean that distance increases which it
does. When you use a car-jack you have to
move your arm a greater distance than if you
lifted up the car straight up with your arms.
Work and Machines Cont’d
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The work you put into a machine to make it
work is called the input work.
Work input = Force input x d input
Work output is the work done by the machine.
Work output = Force output x d output
According the Conservation of Energy Law
energy cannot be created or destroyed. You
cannot get free work done in the machine. In
other words, Win = Wout
Mechanical Advantage
 Machines are NOT designed to increase your
work, but amplify your force. The amount of
force you get compared to the amount of force
you input into the machine is called the
mechanical advantage.
 MA = Fload/Feffort
 Note that mechanical advantage is a simple ratio
and has no units.
 Any machine with a mechanical advantage
greater than one means that the distance you
must move is greater than the distance the
machine will move the load.
Simple Machines
 Device which uses a single force over a distance
to achieve work. Why use a machine to do what
you can do yourself? Either because you want to
increase or change the direction of your force or
distance that you’re moving.
 Eg. An axe is a simple machine which directs
your force along a narrow edge to achieve
greater effect.
 There are traditionally six simple machines. All
other machines are combinations of these six:
The Six Simple Machines
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The inclined plane or ramp
The wheel and axle
The lever
The pulley
The wedge
The screw
I. Inclined Plane
 Device which makes it easier to elevate a load.
The downside is that you must move the load a
greater distance.
II. Lever
 Device that is a rigid bar or plank that can
rotate around a fulcrum. They’re used to
reduce the force needed the force needed
to move a very large load, but again, you
must move a greater distance than the
load will move.
 - There are three classes of levers:
 Measuring forces is hard to do with normal
instruments. It’s much easier to calculate
mechanical advantage of a lever by
measuring the lever and effort arms. To
calculate the mechanical advantage of a
lever, we can also use the following
formula:
 MA = Effort arm/Load Arm