What is a pulley A pulley is a wheel A pulley uses rope that goes around the pulley in its groove The rope attaches to objects Force is applied to the other end of the rope Why use a pulley wheel and not just rope looped around the beam ID: 695073
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Slide1
Pulleys
A simple machineSlide2
What is a pulley?
A pulley is a wheel
A pulley uses rope that goes around the pulley in its groove
The rope attaches to objectsForce is applied to the other end of the rope Slide3
Why use a pulley wheel and not just rope looped around the beam?
Imagine this scene without a pulley.
Can the hay be lifted in some way with just rope?
Why use a pulley wheel? The pulley reduces the amount of friction, making the job easier!Slide4
Pulley
Rope
Force
Weight
1. Fixed Pulley
Fixed Pulley
Object moves
Pulley stays in the same spot
Force applied only on one end of the ropeSlide5
Single Fixed Pulley
Is mechanical advantage provided with this system?
No
The effort needed to lift this load is equal to the weight of the load.
This is because the load is supported by
only 1 rope arm
.
You must also pull an amount of rope equal to the height you wish to lift the load.
This pulley provides the user with
Directional Advantage (allows you to pull down)
, allowing someone to pull down to lift the load up.Slide6
How much effort then would it take to lift this load?
The farmer must use an effort equal to the weight of the load.
Remember, there is no mechanical advantage here…Slide7
Pulley
Rope
Force
Weight
2. Movable Pulley
Movable Pulley
Pulley
moves along the rope
Pulley and object move
together
Wheel supports the load
Rope is attached to something
that does not move
Reduces the force needed to move the object
Reaction
ForceSlide8
Single Moveable Pulley
It requires only ½ the effort to lift the load because there is
more than 1 rope arm
.If the load here ways 20 lbs., only 10 lbs. are needed to lift it.You must pull twice as much rope to lift the load to a certain height.Example: Pull 2 ft. of rope for each 1 foot of height you want to lift the load.
This pulley system provides
mechanical advantage!Slide9
In the second picture, the weight is held by two ropes instead of one. That means the weight is split equally between the two ropes, so each one holds only half the weight.
More rope – More advantageSlide10
We know the facts…but do we know why?
Why is the force needed to lift the load less when you have more pulleys?Slide11
How did you do?
Analogy
: Think of it this way…
If you pick up a load, like a bag of books, that weighs 100 N with one hand, all of the force is directed down one arm. The force acting on the arm is 100 N, counteracting the pull of the books.If you reach down with the other hand and hold the bag of books with two hands, the books still pull with 100 N of force, but the lifting force is distributed between 2 arms. Each arm is pulling with a force of 50 N.Slide12
System of Pulleys
Weight
System of Pulleys
Has at least two wheels
The more complex the pulley, the more the effort needed to move the object
decreases
The force needed to raise this weight is ¼ the weight of the object.Slide13
3. Combined
(double) Pulley
:
This pulley system combines the single fixed & single moveable and their properties giving you bothDirectional & Mechanical AdvantageThis pulley provides directional advantage (allows you to pull down to lift the load)
It also provides mechanical advantage by using 2 rope arms
(which reduces effort needed to lift the load)
In this system, you would pull 2 times as much rope for every unit of height you that you want to lift the load.Slide14
Pulleys are used to gain mechanical
advantage,
trading the amount of rope you have to pull to lift an object for how heavy the object can be. Slide15
The more lines of support (ropes) a pulley has, the more mechanical advantage it has!Slide16
Who has seen pulleys?
…Pulleys are all around us…
Flagpole
Elevator
Window shades and blindsSlide17
More examples
Sails and fishing nets
+
rock climbing gear
+
clothes lines
+
gym training equipment
Cranes
Slide18
Why use pulleys?
Makes lifting things easier
Pulleys
Enables us to use gravity to help us (it is usually easier to pull down to lift something up – directional advantage)Using several pulleys reduces the force required to lift an objectWe have to use more rope and make the rope go furtherMechanical Advantage: More distance traveled, but less force requiredSlide19
Using Gravity
Easier to pull down than up
Elevators use gravity
Counterweight on the otherside of the cableGravity already applying forceon counterweightLess powerful motor required
Counterweight
Elevator PulleysSlide20
Force:
A push or pull on an
object
Fixed pulley: A pulley attached to a fixed point with the rope attached to the object Movable pulley: A pulley attached to the object itself, with one end of the rope attached to a fixed pointMechanical advantage: The advantage gained by using simple machines;
reduced effort
Directional advantage:
Pulleys can change the direction that the effort is applied
Vocabulary & DefinitionsSlide21
Pulley Power!
Single fixed pulleys
–
provide directional advantage you can use counter-weights to reduce your effort but… effort + counter weight must be = or more than the weight of the object being lifted
Single movable pulleys
–
provide
mechanical advantage
you must pull 2 times the distance of rope because the load is supported by 2 rope arms
Compound pulley systems
-
–
provides directional & mechanical advantage
the more rope arms supporting the load the greater the mechanical advantage