Transcript 5 - Pleasant Hill R-III School District

Physical Science
Ch 5 (Part I): Simple Machines
• With your partner, identify the 6 different
simple machines and give 1 example of
each which can be seen somewhere in the
school (not in this room).
.......
• A simple machine is a device which does
work (w = f x d) with only 1 movement.
• Simple machines make work easier by
changing either the size or direction of the
force.
• Does the teeter-totter shown change the
size or direction of the force applied?
• There are 6 different simple machines.
1. Lever
2. Pulley
3. Wheel & Axle
4. Incline Plane
5. Wedge
6. Screw
• If 2 people are pushing identical boxes the
exact same distance, then they are doing
the same amount of work (F x D).
However, if one is pushing the box faster
than the other, then that person is
generating more power.
Power is the rate at which work is done.
• The formula for power is:
Power = Work / Time
The SI unit for power is the watt (W)
• Let’s say Gary is lifting
weights at the gym. It
takes him 2 sec. to lift a
120 N weight over his
head (0.5 m).
• How much work did Gary
do?
• How much power did he
generate?
• A compound machine is a combination of
2 or more simple machines working
together to accomplish a task.
• The force put into a machine to do the
work is called the effort force. That is what
you apply.
• The force then produced by the machine is
called the resistance force.
• So the hand is applying the effort force, and the
resulting force which lifts the rock is the
resistance force.
• Sometimes, a machine can actually produce
more force than you put into it. That’s how
you might be able to lift a person on the
other side of the teeter totter, even though
they may be heavier than you.
• The mechanical advantage is the number of times
a machine multiplies the effort force.
M.A. = Fr / Fe
• “Give me a lever long enough and I could move the world”
Archimedes
• For example:
Bud weighs 100 N. So when he sits on the
teeter totter he is applying a force of 100 N (Fe).
He is able to lift Mike and Gary who have a
combined weight of 150 N. Therefore, 150N
would be the resistance force produced by the
teeter totter (Fr).
The mechanical advantage of the teeter totter
would be, 150 N / 100 N = 1.5
• Randy is using a pulley with a mechanical
advantage of 3 to lift a 5 gallon bucket of
cement which weighs 240 N. How much
force will Randy have to apply?
• Answer: 240 N / 3 = 80 N
• A lever is a rigid bar which pivots about a
fixed point, called a fulcrum or axis.
• There are 3 parts to a lever:
1. Axis
2. Effort arm - where the Fe is applied
3. Resistance arm - where the Fr is applied
f
Resistance Arm
g
Effort Arm
• There are also 3
different classes of
levers. 1st class, 2nd
class, and 3rd class.
• The class is determined
by how the 3 different
parts are arranged in
relation to each other.
• 1st class levers have the fulcrum in
the middle.
• 2nd class levers have the resistance
force in the middle.
• 3rd class levers have the effort force
in the middle.
Workin & Workout
• Work is F x D. The amount of work you put
into a machine (workin) is determined by the
effort force which you apply, and the
distance over which you apply it.
Workin = FE x DE
• For example, if the lever below is pushed
downward 2 m with 50 N of force, then
100 J of work is put into the machine.
That’s workin.
• Likewise, workout is equal to the
resistance force times the
distance which it is moved
(resistance distance).
Workout = FR x DR
• So in the same lever below,
Workout = 25 N x 4 m
= 100 J
• If the amount of work put into a
machine is equal to the amount of
work which comes out of the
machine, then it is said to be an ideal
machine.
Workin = Workout
• However, when a simple machine is used,
often times friction will come into effect.
• For example, if you are using a wheel and
axle which is very rusty, then not only are you
having to overcome the resistance force, but
also the friction created by the rust.
• As a result, you may need to put more
work into the machine to get a certain
amount out.
• The ratio of work in to work out is called
efficiency.
• The formula for efficiency is Wout / Win
• Larry is using a pulley to put some hay up in
the barn. The pulley has an efficiency of .60
(60%), and Larry has to do 150 J of work to
get the hay up in the loft. How much work is
actually being produced by the pulley?
• Answer:
Efficiency = Wout / Win
So, Wout = Efficiency x Win
= .60 x 150 J
= 90 J of work
• A pulley does
500 J of work to
lift a 125 N
weight. How
high was the
weight lifted?
Answers:
• Work = Force x Distance
so, Distance = Work / Force
= 500 J / 125 N
=4
m
1. A rusty pulley has an efficiency of .25. If
a person does 400 J of work to lift an
object, how much work will be done by
the machine?
2. If the person in question #1 does the
work in 8 sec., how much power did the
person produce?
3. As a car travels down the highway, it’s
engine does 186,000 J of work over a
period of 1 min. How much power was
produced by the engine?
4. A kid on a teeter-totter does 800 J of work
as he lifts another kid 1 m upward. If the
other kid had a weight (Fr) of 600 N, what
is the efficiency of the teeter-totter?
5. A crane lifts a 35,000 N steel girder a
distance of 25 m in 45 sec. How much
work was done, and how much power did
the crane require to lift the girder?
6. How many kilowatts of power were
produced by the crane in #5?
Kim and Chris want to teeter totter
together. However, Chris weighs 25
pounds more than Kim. If Chris sits 2 m
away from the axis on her side of the
teeter totter, will Kim need to sit the same
distance away, closer to the axis, or
further back from the axis? Why?
On a typical trebuchet, the resistance arm (projectile
side) is longer than the effort arm (counter-weight
side). However, if the effort arm were made longer
and the resistance arm shorter, that would increase
the amount of effort force put into the machine.
Would this result in the projectile flying further?
Explain.
• Identify 4 simple machines used in the Goldberg
Device below, and tell why it would probably not work.
• Come up with 1
completely new
example of each of the
3 different classes of
levers.
http://www.sciencenetlinks.com/interactives/powerplay.html
• Pooh (140 N), Eeyore (160 N), Tigger (155 N),
Rabbit (120 N), and Rooh (25 N) are trying to
teeter totter with Heffalump (7,200 N). How
much of a mechanical advantage will they need
in order to teeter totter?
• After reading up on some basic physics,
Rabbit determines that the necessary
mechanical advantage which they figured
out did not work for a couple of reasons.
What were they?
• Strength and power are not the same
thing. Do you think it’s more important for
an MMA fighter to have more strength or
more power? Explain.
Subject #
Weight
(lbs)
Weight (N)
Distance
Traveled
Time (1)
Time (2)
Average
Time
1
2
3
1. How much work was done by each of the subjects? (Show work!)
Subject 1 _________
Subject 2 _________
Subject 3 _________
2. How much power was produced by each of the subjects? (Show work!)
Subject 1 _________
Subject 2 _________
Subject 3 _________
3. Did the person who made it up the stairs the quickest produce the most power?
Why or why not?
4. Do you see any relationship between the size of the person and the amount of work
done? If so, what is that relationship?
5. Do you see any relationship between the size of the person and the amount of power
produced? If so, what is that relationship?