[Buoyant Force]
Dan: What is all this stuff here you got,
JJ?
What is this weird looking can thing with
a hole on it?
JJ: This is the overflow can and what we use
this for is to capture displaced fluid.
The can is filled with soapy water to this
spout level here.
If we put anything or we submerge anything
down in the fluid, it's going to displace
some out into this beaker here.
We're going to catch it in that beaker.
Dan: If we put this whole cylinder in, then
the volume of the water that goes into this
beaker would equal the volume of the cylinder.
JJ: That's correct.
Yeah.
Dan: Now what if we want to know the mass
of that water?
JJ: What we're going to do in this experiment
is we're going to measure a few things.
One of them is the mass of the displaced water.
I've got a balance here that we're going to
first put the beaker on.
I'm going to actually do that right now, so
we can use that beaker.
Dan: You can either record the mass of the
beaker which is about 50 grams or...
JJ: Right.
What we're going to do is we're going to put
the beaker on here, and then we're going to
tear this or zero it, and then take it off.
Although the scale reads a negative value
right now, if we put fluid in the beaker and
then put it back on, it's going to tell us.
It subtracts out automatically the mass of
this beaker.
Dan: We have to do less math that way.
JJ: Yes, that's right.
Dan: That's why you want to do it.
JJ: That's right.
The balance is going to tell us the mass of
the fluid that gets displayed.
Dan: Now, why are we hanging the cylinder
from a smart card?
JJ: What we're going to do in this experiment
is we're going to submerge this metal cylinder,
and we're going to measure the tension in
the string here from the weight of the cylinder.
We're suspecting that tension is going to
change as we submerge it because of a buoyant force.
We're trying to see how that change happens
depending on how much of the cylinder is submerged,
and how much water gets displaced.
We're going to measure the force of tension
here, while also comparing the weight or the
mass of the water that gets displaced.
Then we're going to use that to calculate
the weight of the water.
Dan: So the upward force of tension, you're
saying, is going to go down as we submerge
it, and the amount it goes down is the buoyant
force.
JJ: That's right.
Right now, the force sensor is measuring the
tension, and that tension is due to the weight
of this brass cylinder.
As we submerge it, the measurement that the
sensor is making, the tension is going to change.
Well, we suspect it will change.
That change will be that force from the buoyancy.
Dan: I need to record what the tension is.
Right now, it's 2.01.
We should make sure it's zero and that's not
on it.
JJ: In this experiment, one of the first things
we want to do is make sure we zero our four
sensors.
Well, nothing's there.
Dan: It's zero.
JJ: So we're pretty good there.
Again, I'll let go and that tension that it's
measuring is the weight of that brass cylinder.
Dan: I'll enter that.
JJ: Yeah, and so that value on the right side
of your screen, that's a live reading from
the sensor, and that tells you the tension
in the string.
Dan: Now, it says here in the data table,
we're going to submerge it one quarter its length.
JJ: Yes.
In the experiment, we're going to collect
five data points.
One when it's not submerged at all.
One when it's a quarter, one when it's half
submerged, when the half of the length of
it is submerged, three quarters, and then
completely submerged.
Dan: We'll need to know that total length.
I'll enter that in the data table as the depth
when the whole thing is submerged.
Each one of these cylinders is 6.5 centimeters.
Enter that because you will need to know that.
JJ: Yeah, and so what I've done here, and
what the experiment has you do is you take
your cylinders, and these two cylinders are
identical in terms of their geometry.
Then you measure and mark the cylinder in
quarter sections.
There's three marks on this cylinder indicating
one quarter, half, three quarter and then,
obviously, if it's fully submerged as well.
Dan: I can see this one has the same thing,
so that will help with the data collection.
JJ: Right, and so we're going to go down in
stages.
First, with none of this submerged, a quarter
half, three quarters, and then fully.
Dan: Now it looks like this might be tilted
a little.
Is that going to cause a problem?
JJ: Yeah.
We want to make sure that the cylinder hanging
from the center is hanging straight up and
down, because the volume that we're going
to submerge is part of this experiment.
Dan: It'd be easier to calculate.
Again, less math.
JJ: Yes.
If we submerge a cylinder straight up and
down, we know that we can easily figure out
what that volume is.
Dan: OK, let's do it.
JJ: We've zeroed the balance.
The water level is at the right height.
When we submerge this, fluid will displace
and go into that beaker.
I think we're ready.
No mass has been displaced now.
I'm going to loosen this and then lower it
down until a quarter of the cylinder is submerged.
Dan: Yeah, water's coming out.
JJ: All right, and I'll just lock that in.
This dripping can take a while.
We might speed things up a little bit in this
video just to help because we're going to
do this three more times.
Dan: OK.
[pause]
JJ: The dripping has slowed down quite a bit.
The drips just get further and further apart.
Once they get far enough apart...and what
did you say?
Once every 15 seconds or so?
Dan: Yeah.
JJ: Then that's probably good enough.
Dan: That's one more coming out.
I'd say if you can measure the mass of the
water before the next group comes out, then
it's probably good.
JJ: I'm going to take this, put it on our
balance, and it reads 5.26 grams.
Dan: Enter that and then the tension definitely
went down.
It's fluctuating between 1.96 and seven, or
at least it was.
Now we're ready for submerging it halfway.
JJ: Right, so this and that.
There's halfway and the dripping commences
again.
Again, the dripping is slowed quite a bit.
Well, maybe wait for one more drop and then
make our mass measurement real quick and record
our tension value.
Dan: Sometimes you can speed it up by tapping
it a little bit, but you don't want to upset
it if you want accurate data.
That's good.
JJ: I'm going to put this on the balance,
11.24 grams.
Dan: That makes sense.
It's a little more than double the other one.
Then we have 1.9 tension.
We're ready for three quarters.
JJ: All right, three quarters and the dripping
goes again.
We'll let this drip go in and then maybe one
more, and then we'll go ahead and measure
the mass.
There we go.
That will get that out there.
As soon as it drips, we'll go ahead and take
it.
This mass is 16.76.
Dan: Get that entered, and the tension is
1.84.
Now, we're ready for three quarters.
I mean all the way.
JJ: Fully submerged.
Dan: One, three quarters.
[pause]
JJ: Dripping has slowed again.
One about every 15, maybe actually more than
15 seconds.
We'll wait for this last drop to go in, and
then we'll take this last data point for the
cylinder.
And drip.
The mass is 23.95.
Dan: The tension 1.77.
The remaining things in this data table include
figure out the volume that's submerged.
Now the entire thing is submerged.
You know its length and it's a cylinder, but
you also would need to know the radius.
We're going to tell you the radius here.
There's not a place in the data table.
The radius of the cylinder is 1.17 centimeters.
You need to know that to be able to figure
out the volume.
JJ: Right, and then once you know the volume
of the whole cylinder, it's just some easy
math to figure out what the volume is when
you had a quarter of it, half of it, and three
quarters of it submerged.
Then we're going to repeat this experiment
using the aluminum cylinder, and we're going
to follow the exact same procedure.
We're just going to provide the data that
we collected for that separately.
It'll be in with the brass cylinder data when
you go into the file.
Dan: Aluminum cylinder, what would be different
about that steel metal?
Why would we do that?
JJ: That's correct.
The densities of the two metals are different.
Dan: I wonder, would that mean the tension
would go down more or less?
JJ: Well, that's what we'd find out in the
experiment.
Observe your data and answer the analysis
questions accordingly.
