What is a pulley?

A pulley is a wheel A pulley uses rope that

goes around the wheel (often, in a groove)

The rope attaches to objects The other end of the rope has

a force applied– Applied force is a push or a pull

Who has seen pulleys?

…Pulleys are all around us…

Flagpole

Elevator

Window shades and blinds

More examples

Sails

and fishin

g nets

+ rock climbing gear

+ clothes lines

+ gym training equipment

Cranes

Why use pulleys?

Makes lifting things easier Pulleys redirect force

– Enables us to use gravity to help us (it is usually easier to pull down to lift something up)

Using several pulleys reduces the force required to lift an object– We have to use more rope and make the rope

go further

Joe’s Problems

#1 Joe’s Got Problems…actually just one BIG problem.

Hi again!His problem is his pooch, Rex.He’s one sick puppy.He’s also freakin’ HUGE!

#2 At the vet’s office…Joe has to get Rex uponto the examinationtable.

Yikes!

#3 Joe’s been working out…

…but not enough. Rex is justtoo heavy to lift.

#4 Joe learns his lessons well!Yo, uncle! WhaddooIdo?

Use a PULLEY, Joe!

I’m busy, so I gotta go!

#5 But Joe’s new at Pulleys

…so he does what anybody should…a little research!

Pulley – arrangement of wheel(s) and rope thatreduce the amount of force needed to raise an object.

#6 He tries the first oneFixed pulley – uses one wheel to change only the directionof force.

ceiling

A “fixed” pulley wasn’tbroken and is now repaired.“Fixed” in this sense refersto the fact that the wheelis attached to the ceiling andspins in place.

#7 Joe Pulls…

#8 And……nearly breaks his back.

Oh yeah! Joe remembersseeing that a fixed pulley justlets you pull down on rope insteadof pulling up on the object.It doesn’t make Rex feel one ouncelighter!

People do use fixed pulleys because pullingdown on rope is easier on the body thanbending over to pick the same weight up.

#9 Joe tries a different pulleyA Running Pulley – uses a wheel hung on a rope to pull upwards,reducing the force by half.

#10 Joe Pulls……and starts to lift Rex off the ground!

#11 But there’s one issue……Joe can’t get Rex high enough to get on the table.On his tippy toes, Joe’s just not tall enough to keep pullingup and up and up on the rope. Rex only makes it halfwayto the table top.

#12 Joe hatches a plan…He realizes that both the fixed pulley and movable pulleyhave advantages and disadvantages…

The fixed pulley doesn’t make Rex feel any lighter, but theRunning pulley did allow Joe to lift his sick pooch off theground.

The problem with the running pulley was having to keeppulling up, up, up. However, the fixed pulley does allowyou to pull down on rope.

#13 Hmmm…

#14 The Plan!Combine the ADVANTAGES of the fixed and runningpulleys without suffering the DISADVANTAGES ofeither!

#15 Joe’s Research

Joe uncovers a way to string this all together so that it helpshim get the job done!

Block and Tackle Pulley – a combination of fixed & movable Pulleys that reduces the force by half

#16 The Block and Tackle Pulley at Work

#17 Joe Pulls……and starts to lift Rex off the ground!

And the good news is that he is able to pull down on the ropeso that he can just pull, pull, pull. Rex goes up, up, up.

#18 And Joe Has Just Accomplished…

•Reducing the amount of force it took to do work.•Making sure he got to pull in a direction that’s easy for humans to do.•Yes, he had to pull more rope, but that’s a trade-off worth making.

But most importantly…

This was possible due to mechanical advantage. Themovable and double pulleys make work feel easier!

#19

ARF!

#20 One Final Point:As Joewas doinghis researchherealized…

The more wheels & ropes, the better the pulley. You have topull a ton of rope, but can easily lift heavy objects.

#21 And That’s…