You work for a construction company, and you need to be able to lift a bundle
of bricks from the ground up to the top of a building.
You suspect that you'll need to use a pulley for this, but the question is,
how many pulleys do you need?
Will one be enough?
Is two better?
Maybe three?
What we're going to do in this activity is, we're going to measure the force
required to lift something that represents the bricks, and see which one of these
systems is better for this problem.
This weight is going to represent the bricks that we need to lift to the top
of the building.
I'm going to attach this force sensor to the bricks.
In every case, we're going to lift these bricks the same amount.
So over here, I have a ruler.
Let me lift the bricks up 5 centimeters, and hold them as steady as I can.
You look on the screen, it looks like the weight of the bricks is
about 4.9 newtons.
Okay?
So let's record that, and keep that handy for later.
So here I have a system that has one pulley, and then what I'm going to do is
I'm going to take the force sensor, and I'm going to connect it to
the other string.
Let's lift them 5 centimeters.
The system is asking, how many strings are holding up the weight?
If you notice, one string is actually holding up the weight of the bricks.
So I'm going to type in one.
If you look at the data, you'll see that the force applied,
4.79 newtons is very close to the original weight of the bricks.
But in this system, you'll notice that the force was applied in the
opposite direction that the weight moved.
So this single pulley changed the direction of the force.
What happens if we have the weight lifted by two strings.
We have a string up here where the pulley was before.
The pulley is free to move.
And well, there's another string here.
Both of these strings are attached to our bricks.
Let's lift the bricks the same amount, 5 centimeters.
The system is asking, how many strings are holding
up the weight?
Well, in this case, there are two strings holding this weight up, so let me
type in two.
And then let's take a look at that data.
The original weight was 4.9 newtons.
With 1 string, the force applied was 4.79 newtons, but with 2 strings,
the force that we have to apply is 2.6 newtons.
The weight hasn't changed, but what's happened now, two strings are sharing the
weight of the bricks, so this is requiring us to apply less force.
\What happens if we increase the system to three strings?
We have three strings holding up our bricks.
Let's do the same thing we did before.
So every time we're going to lift the bricks 5 centimeters.
You may notice that I'm pulling this outer string further than ever.
So let me lift that up 5 centimeters, and hold it up as steady as I can.
Now I have three strings holding up the weight.
You can see that with one string supporting the bricks,
the only effect was to change the direction of the force.
The amount of force applied was nearly the same as the weight of the bricks.
With two strings holding up the bricks, each string held about half the weight
of the bricks, and so we only had to apply half the force.
We paid for that by having to pull the string further.
In the case of three strings, we applied a fraction of the original
weight of the bricks.
The bricks' weight didn't change.
The amount of work done on the bricks to lift them was always the same because we
always lifted the same bundle of bricks 5 centimeters.
However, in the third case, we applied only 1.6 newtons of effort
force to lift the bricks the 5 centimeters.
So we had to apply the force over a larger distance.
We did three pulleys, two pulleys, one pulley.
What if we did more pulleys.
If the system was more complex, would that make it easier
to lift the weight?
Would it make it more efficient if we used more pulleys?
Something for your students to try out.