Pulleys A simple machine
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? • 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!
1. Fixed Pulley Pulley Force Rope • Fixed Pulley • Object moves • Pulley stays in the same spot • Force applied only on one end of the rope Weight
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.
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…
2. Movable Pulley Reaction Force Rope Force • 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 Pulley Weight
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!
More rope – More advantage 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.
We know the facts…but do we know why? Why is the force needed to lift the load less when you have more pulleys?
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.
System of Pulleys • 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. Weight
3. Combined (double) Pulley: • This pulley system combines the single fixed & single moveable and their properties giving you both • Directional & Mechanical Advantage • This 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.
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.
The more lines of support (ropes) a pulley has, the more mechanical advantage it has!
Who has seen pulleys? …Pulleys are all around us… Elevator Flagpole Window shades and blinds
More examples Cranes Sails and fishing nets + clothes lines + gym training equipment + rock climbing gear
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 object • We have to use more rope and make the rope go further • Mechanical Advantage: More distance traveled, but less force required
Elevator Pulleys Using Gravity • Easier to pull down than up • Elevators use gravity • Counterweight on the otherside of the cable • Gravity already applying forceon counterweight • Less powerful motor required Counterweight
Vocabulary & Definitions 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 point Mechanical advantage: The advantage gained by using simple machines; reduced effort Directional advantage: Pulleys can change the direction that the effort is applied
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