Transcript Unit 14: Work and Simple Machines

Unit 9: Work and
Simple Machines
The Meaning of Work
In science you do work on an object when you
exert a force on an object that causes the
object to move some distance.
Example: pushing a child on a swing
No Work Without Motion
In order for you to do
work on an object, the
object must move
some distance as a
result of your force
In other words: you can
huff and puff, but if
you don’t blow that
house down, no work
has been done.
Only Force in the Same Direction
In order to do work on an object, the force you exert
must be in the same direction as the object’s
motion.
If the force is NOT in the SAME direction the object is
moving, no work is done.
NO WORK
YES, WORK
Work or No Work?
Test your understanding of work so far?
1) A snowstorm hits town and you go help your parent push your
car out of the snow. You push as hard as you can but the car
doesn’t budge NO WORK
2) You carry a heavy bag of books to school everyday NO WORK
3) You lift the bags of groceries from the cart into the trunk WORK
4) You pull two kids on a sled WORK
5) You pull the books out of a back pack WORK
6) You hold a long piece of wood up at one end, while someone
hammers the other end NO WORK
Work is…
1)A force is exerted causing the object to move
AND
2) The object is moving in the direction of the
force
Calculating Work
The amount of work you do depends on both
the amount of force you exert and the
distance the object moves:
Work = Force (x) Distance
W = F (X) D
OR….
W
F
D
Work is Measured in Joules (J)
Force is measured in…
Newtons (N)
Distance is measured in…
Meter (m)
The SI (Systeme International) unit for work is the
newton (x) meter = Joule (J)
One Joule is the amount of work done to exert a force
of 1 newton to move an object 1 meter
Why do we use machines…?
To make work easier!!!
A machine makes work easier by…
changing the amount of force you exert,
the distance over which you exert the force, or
the direction in which you exert force
In other words: a machine takes the effort
required to move an object from point A to
point B easier
Force and Machines
When a machine does work the force exerted on the machine is
called the…
Input Force
The machine exerts a force over some distance, the force
exerted by the machine is called…
Output Force
Mechanical Advantage
When input force is compared to output force
you are determining the advantage of using a
machine.
A machine’s mechanical advantage is the
number of times a force exerted on a machine
is multiplied by the machine
Consider it this way…
Mechanical Advantage (MA) = Output Force ÷ Input Force
Scenario 1: Can Opener
Scenario 2: Piano
You exert 20 newtons of force
on an can opener which in
turn exerts 60 newtons on
the can.
60 ÷ 20 = 3
Output Input MA
The can opener triples the
work done!
It would take 3,200N to lift a
piano up on to a stage.
Instead you use a ramp
which requires 1,600N.
3,200 ÷ 1,600 = 2
Output Input MA
The ramp doubles the force
you exert.
Efficiency of Machines
Ideally, we would want to put in a minimal
amount of input force and get out multiple
amounts of output force.
However, we know this isn’t true.
Some work is wasted overcoming friction,
tightness of parts, or the “design” of machines
-Poor design!
Simple Machines
There are six basic kinds of
simple machines:
1)
2)
3)
4)
5)
6)
Pulley
Wedge
Wheel & Axel
Inclined plane
Lever
Screw
Remember this sentence
Paul
Wants
Work
In
Life to be
Simple
1) Pulley
Grooved wheel in
which a rope,
chain, or cable is
pulled to raise an
object.
Examples: Window
blinds, flagpole
2) Wedge
A triangular devise used to split or
push objects apart
A wedge can be considered two
inclined planes
Examples: Axe, zipper
3) Wheel & Axel
Two circles that are attached to one another and that rotate
about a common axis. Used for transporting of objects.
Examples: Cars,
Door handles
4) Inclined Plane
A flat, slanted surface used to lower or raise objects. The longer
the ramp, the more mechanical advantage.
Example: hallway ramp
5) Lever
A rigid bar that is free to
pivot, or rotate about a
fixed point. A lever works
in two ways:
1) it increases the effect
of input force and,
2) it changes direction of
the input force
Examples: Spoon, door
handle, crow bar
6) Screw
An inclined plane wrapped
around a cylinder. Used to
connect to objects
The closer the thread on any
screw the greater the
mechanical advantage
Examples: Faucets, jar lids
Rube Goldberg Machines
A Rube Goldberg machine, contraption, invention, device, or apparatus is a
deliberately over-engineered or overdone machine that performs a very
simple task in a very complex fashion, usually including a chain reaction. The
expression is named after American cartoonist and inventor Rube Goldberg
(1883–1970). -Wikipedia
Examples:
Bowling Example(good)
Better
Rube meets simple machine
And the greatest mix of science and music(best!)