[Captions by Mary V. at Y Translator] In today's video, we're going to be taking a look at pulleys.
One of the classic six simple machines.
We'll take some measurements, demonstrating how they work,
and then we'll try building a scale model of an electricity free elevator.
A pulley is one of what renaissance scientists
referred to as the six basic machines.
Six mechanical devices that helped you change the direction
or magnitude of a force.
The other five are a lever, a wheel and axle, an inclined plane, a wedge and a screw.
Today, we'll be looking at pulleys and taking some measurements
with this fish scale to see what happens to the different weights
on a rope as it runs through one or more pulleys.
After that, the plan is to build a scale model of something I came up with
a few years ago and maybe other people have come up with it
hundreds of times before me.
But the idea is to make an elevator that uses just weights, ropes
and pulleys and no electricity.
Here's the idea.
A fixed weight will have two separate pulley systems attached to it.
One that makes it so you could ride the other end of the rope up.
And one that makes it so you could write it back down.
First off, we need a frame that we can attach stuff to.
So I've got a bunch of scrap wood and we're going to start
throwing some stuff together.
[Music]
This is not the most official-looking construction ever.
Some pieces are kind of twisted and warped and it's just not flat.
That's because this is literally just thrown together from pieces
that I already had just in my scrap pile but it should work pretty well.
The important thing is that this end over here can take a good amount
of weight and we can do our tests on this.
Here's the first pulley we're going to do some tests with.
It's designed to just be permanently attached onto something.
It doesn't move around like some other kinds of pulleys.
It just stays in one spot but that's exactly what we want at first.
[Music]
We've got our pulley attached and we've got a thin cord
running through the pulley.
We're now going to attach that on to this little clip.
That's just to make things a little bit easier
as we're disconnecting and reconnecting stuff.
We have here a fishing scale.
We've got here what is supposed to be a 10-pound weight.
This is just sort of a barbell exercise weight.
We're going to just hook that on with this little rope and see
what measurement our scale gives us.
10.17 pounds for those who are curious.
That is 4.6 kilos,
so we can see just over 10 pounds.
It could be that the weight has a little bit of extra on it.
It could be that this fish scale is not 100% accurate but it's pretty close.
It's supposed to be 10 pounds.
It's showing up as almost 10 pounds.
Now let's see what happens if we connect this to our pulley system
and then use our scale to pull on that.
All right. Let's add a little bit of a loop into our rope right here so that we
can hook our scale on to it.
Pull on the loop, weight goes up.
All right turn our scale on.
The scale is going to be upside down,
but we should still be able to see the numbers.
So it was 10.17 pounds.
10.17.
We got exactly the same read out.
A single fixed pulley does change the direction of the force
but it doesn't change the magnitude of the force.
I've now got a 7.5 pound weight with a hook attached to it,
and we're going to put that through the loop instead of our scale,
and what we're going to see is that it's not heavy enough to pull the 10-pound weight.
Of course it's not.
10 pounds is more than 7 and 1/2 pounds.
Drop it the other way.
We can see that the 10-pound weight is of course enough to
lift the lighter weight.
With our 7 and 1/2 pound weight attached to one end, if we now
attach the fish-scale to the same spot, it's not going to take as much
pull because it's already got the weight.
Same deal.
We've got static friction coefficient to overcome but we can see that what
we're getting on the scale varies between about one and a half pounds
and well I think we got up to 6 there.
But an equilibrium does seem to be at about two and a half pounds,
which is exactly what we would expect to see.
Let's change things up and try attaching a single movable pulley to our
weight instead of one fixed pulley above.
We've got our 10-pound weight, we have a pulley attached
to the top of the weight.
On one end, our cord is just attached to the beam.
It's not running through this pulley.
It's just on the beam.
Separate from that.
And the other side we can just pull with our hands.
So how hard are we going to have to pull on this rope in order to get
that weight to lift off the ground?
Well let's look at it.
If it's just suspended in the air.
We know we have 10 pounds of weight hanging down,
and we've got two cords,
that means each cord should have five pounds on it.
And that's not really going to change.
Even if we're pulling it up and down.
We should still have five pounds are suspended in this cord,
and five pounds suspended in this cord.
Here we go.
Here's our scale.
Pull till the weight starts lifting and we are at about 5 pounds.
We pull really fast for a second.
It might register as even higher but for the most part we get
approximately five pounds of force necessary to lift this 10-pound weight.
Now with our single movable pulley system it cuts the weight down in half
but of course there has to be a trade-off.
You can see that if I move my hand, I'm grabbing the rope right at the pulley.
If I lift it from here, all the way up to this beam,
the weight didn't move the whole distance to the beam.
The weight moved half of the distance to the beam.
So it only takes half as much force to lift this weight off the ground
but it only moves half as far.
There has to be a trade-off and you can't remove the amount of work it takes
to get that weight up to its final height.
And see to raise it all the way up,
my hand has to travel twice the height of the weight.
But now we have an issue back to where we first started before we'd even
come up with the pulley in the first place is that we're lifting up.
Sure it's easier to lift up than it was before but we're still pulling up
which is not as easy of a motion as pulling down.
So we can run it through the fixed pulley again.
And now we can pull down.
And lift the weight up.
The fixed pulley doesn't change the magnitude
but it does change the direction.
So this takes five pounds pulling up.
That means this takes five pounds pulling down.
And of course if we move the entire distance from the top pulley
to the bottom pulley,
our weight moves approximately half that distance.
I have to go back and pull it a second time to get it all the way up.
I've got a couple more pulleys so I'm going to add another one to the top,
another one to our weight and see if we can reduce how much we
have to pull on this even more.
All right.
The cord starts fixed on one end of the beam.
It's going down through the first pulley up through the fixed pulley on the top.
Now we're going to come back down through the second movable pulley
on the weight.
And then we're going to go back up to this newly fixed pulley.
It's feeling pretty lightweight to me.
Let's check how much it takes to move this with two moving
pulleys and two fixed pulleys.
It's showing up as anywhere between one and a quarter pounds,
and almost four and a half to overcome friction.
But what we should be looking at is one quarter of the total weight
that would be two and a half pounds.
Previously we attached our seven and a half pound weight
to a single fixed pulley system.
And of course the 10-pound weight lifted it right up.
But now we've got a system of moving pulleys,
and we should see the opposite effect.
Quite a bit in fact.
There is very little resistance there the seven and a half pound
weight just plummets to the ground
lifting the 10-pound weight 1/4 of the distance to the pulleys at the top.
Several years ago while I was just doing some doodles, drawings,
messing around with how pulleys systems work,
I thought of this idea that you
might be able to use pulleys and weights
to build a sort of elevator.
The concept I thought was pretty fun.
And so what I wanted to do is build a scale model.
And it's not just one weight that would lift you back up.
It's a weight that has two separate systems attached to it.
One where the weight is attached to a single fixed pulley,
so the weight would pull you up,
and another system run at the same time
that has a movable pulley and a fixed pulley.
So your weight would overcome the counterweight,
and you could ride it back down.
[Music]
We have our system all set up.
We've got a 10-pound weight with two separate pulley systems attached to it.
The black one is a single fixed pulley so it's not
changing the magnitude.
It's still an even ten pound pull.
But it is changing the direction.
The weight goes down, it pulls the rope up.
The movable pulley changes our magnitude dividing it in half
and the fixed pulley changes the direction
so that it's still the opposite direction as the movement of the weight.
So imagining that this whole thing is a scaled-down tree-house,
this weight represents the weight,
this 7 pound weight represents the person or you know, we'll say me,
and this represents the little ledge in the treehouse
of the treehouse itself.
This is actually non-functional.
It's just there to prove a point.
The black cord which is on the fixed pulley and that is currently
tied to something on the ground.
And this little loop is where our person can grab a hold.
Now as we untie the rope from where it's wrapped around this screw,
the heavier weight takes over.
Lifting this weight well almost all the way up to this platform.
Like I said this is just illustrative.
We can just say that they can climb on from there.
Yay!
We have climbed up onto our platform.
Just as we wanted.
But now you need to get down in the weight is all the way down there.
So what you do, is you clip into the other system,
and climb out of the treehouse.
You are now gently lowered down to the ground by the pulley system and the weights.
Of course the weight is now only halfway there in the loop to
get back up is way out of reach.
So what you need to do is just keep pulling on the orange cord,
lift your weight all the way back to the top until you can reach
the loop for the other side again.
This is actually one set up where you would be glad that the pulleys
aren't 100% frictionless.
If they were perfect pulleys then as you jumped out of the tree house,
you would actually still drop at the full speed of gravity.
You would hit the ground holding a rope that took a whole bunch of your
weight off so it wouldn't be like hitting the ground full speed
normally jumping out of the tree house.
Your legs wouldn't be supporting your whole weight but it'd still
probably be pretty scary to be rushing in at the ground that quickly.
Overall, the idea does work pretty well and I hope that I get a chance
in the future to try this out as a full-scale treehouse elevator.
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