Video Transcript
In this video, our topic is drawing
conclusions. Whenever a scientific experiment is
performed, coming to a conclusion is an important part of that process. As we’ll see, a conclusion that’s
well-established is one that’s based on the experimental results and the hypothesis
for the experiment.
Scientific experiments begin with a
hypothesis. This is a prediction we would make
about some physical phenomenon. An example of a hypothesis is if we
drop a lead ball and a feather from the same height at the same time, then the lead
ball will reach the ground first. This is our prediction. And to test this prediction, to
test the hypothesis, we conduct an experiment.
Experiments are carefully designed
to investigate the issue we’re interested in. Once an experiment has been
conducted, it leads to outcomes, also called results. These are qualitative or
quantitative observed or measured values. In the case of our lead ball and
feather being dropped at the same time from the same height, a qualitative
observation may be that we see the lead ball reaching the ground first. A quantitative value would result
if we measure the time each object took to reach the ground.
Based on our experimental outcomes
as well as the hypothesis we formed, we’re able to draw a conclusion. Formally put, this is a statement
that can be accepted as correct based on the results of an experimental test of a
hypothesis. To get a better sense of how this
all fits together, let’s imagine an experiment we might conduct.
Suppose that one day we found
ourselves with a cylinder of helium gas and an uninflated balloon. Knowing that balloons filled with
helium are lighter than air, say that we attach the balloon to the nozzle of the
cylinder and fill it with helium gas. Say further that we release the
balloon and watch as it rises. After rising for some amount of
time, eventually the balloon goes out of sight. Watching this happen, we start to
think.
We know that the air in Earth’s
atmosphere is more dense, it’s more tightly packed, toward the surface of the
Earth. And as we go up in altitude, that
density decreases. We realize that the reason the
balloon rose was that, being filled with helium gas, its density was less than the
density of the air around it. But we continue to think if the
balloon kept rising higher and higher into less and less dense atmosphere, then
eventually it should reach some altitude where the balloon’s density and the density
of the air around it is the same. At this point, we expect the
balloon to stop rising and its elevation to remain constant.
Based on this, we form a
hypothesis. This is a testable prediction. We hypothesize that a helium-filled
balloon will ascend until its average density equals the density of the surrounding
air. With this in mind, we then go about
designing an experiment to test this prediction. We get a bag of balloons. And say that we also set up a video
tracking display that will show the balloons as they ascend through kilometers of
atmosphere.
On this display, we have, marked
out in a green dashed line, the altitude we believe the balloons should achieve in
order for their density to equal the density of the air around them. Based on our predictions, the
balloon should stop ascending at this green line. That’s the design of our
experiment. And our next step is to fill up and
then release some helium-filled balloons.
We grab a balloon from the bag,
fill it up with helium gas, and then let it start to rise upwards. Soon, the balloon appears as a dot
on our video tracker. And we see the dot get higher and
higher over time. As we watch the balloon rise on the
tracker though, before it reaches the green line marked out as where we predict the
density of the balloon will equal the density of the air around it, we see our dot
disappear.
We’re surprised by this and not
quite sure why it happened. But in any case, we move on to
trial number two. We fill up a second balloon with
helium, release this one, and again watch as it rises, first when it’s close to us,
and then on the screen as it gets farther away. Like the first balloon, this one
ascends for some time. But then, before it gets to the
green line of elevation, it also disappears from view.
Now, say that we repeat this
process three more times for a total of five launched balloons. And say that all five of them
disappear from view on our video tracker before they reach the elevation we expect
them to reach. As we look at the final locations
of each one of our balloons before they disappeared, we’re viewing the results of
our experimental test. We can summarize these results by
saying that all the balloons released failed to reach the altitude predicted. That is, none of them reached the
altitude where their calculated density was equal to the density of the air around
them.
Now, based on our hypothesis and
based on the results of our experiment, we can form a conclusion. Our conclusion is that balloons
filled with helium do not ascend to an altitude where their average density equals
that of the surrounding air. Now, this may not seem like the
world’s most interesting conclusion. We see that it’s essentially a
nullification of our hypothesis.
But recall our definition for what
a solidly founded conclusion to an experiment is. It’s a statement we can accept as
correct based on the results of a test of a hypothesis. And indeed, based on our hypothesis
and the results of our test, we can accept this conclusion as correct.
Imagine, though, that in our
conclusion, we went beyond the bounds of the results of our hypothesis test. Say that we not only claim that
helium-filled balloons don’t ascend to the altitude where their density equals that
of the surrounding air, but that we also attribute a reason for that. Say that we add on to our
conclusion that the reason these balloons don’t rise to the expected altitude is
because they’re destroyed by birds. Now, it may or may not be correct
that these balloons are destroyed by birds as they ascend. But either way, we can’t include
this in our conclusion because it’s not supported by the results of our experiment
or our hypothesis.
Now, note that even if we constrain
our conclusion to information in our hypothesis or our test results, that doesn’t
guarantee our conclusion will be correct. For example, maybe we
misinterpreted our results. Maybe the balloons disappeared from
our tracker not because they were destroyed, but because our tracker somehow
malfunctioned. In that case, our results wouldn’t
reflect what truly took place. And then our conclusion wouldn’t
either.
In general, in a scientific
experiment, some human interpretation is involved. This interpretive element may lead
us ultimately to draw a conclusion that is not correct. But one thing we can insist on and
execute correctly is basing our conclusions on the results of our experiments and
our hypotheses. When we do this, we establish an
internal coherence across our experiment. Knowing all this, let’s now get a
bit of practice with these ideas through an example exercise.
Newton’s first law of motion states
that an object in motion will not change its speed unless something pushes or pulls
the object. A student gives a single, short
push to a ball and the ball rolls along flat ground. The student observes that the ball
slows down but does not see anything push or pull the ball. Which of the following conclusions
that the student might arrive at is correct?
Okay, before we get to these
conclusions, let’s get a picture of what’s taking place here. Here, we have a student who is
standing next to a ball that’s at rest on some level ground. The student gives the ball a short
push. And this sets the ball in
motion. Interestingly, though, the ball
starts to slow down. But all this time, the student
doesn’t see anything push or pull the ball. Given our understanding of Newton’s
first law of motion, this seems a bit strange. Now, let’s look at some possible
conclusions that the student might draw.
The student might conclude that (A)
Newton’s first Law of motion is incorrect. (B) The ground is not as flat as
the student originally thought, so the ball was actually rolling uphill. (C) The ball was pushed or pulled
by something that the student cannot see. (D) Earth’s motion around itself
opposes the ball’s motion.
Let’s consider these possible
conclusions one by one, starting with option (A). Recall that Newton’s first law of
motion says that an object will maintain its motion unless it’s acted on by an
unbalanced force. In other words, it’s given a push
or a pull. This law is one of the most
carefully tested laws of motion there is. In this instance of the student
noticing the ball slowing down even though it’s on a flat surface, it’s possible, at
least theoretically, that this is an indication that Newton’s first law of motion is
incorrect.
But when we entertain that idea,
it’s important to consider the evidence on both sides. There is, in some cases, quite
literally, a mountain of evidence supporting Newton’s laws of motion. Compared to that mountain, this
single observation the student is making is like a grain of sand, so small by
comparison, that if we had to pick one or the other to trust in, we would pick
Newton’s first law. Since, as we’ve seen, this law is
so well-supported, we won’t choose option (A) as a correct conclusion that the
student could draw.
Next, let’s think about the
conclusion that the ground is not as flat as the student originally thought so the
ball was actually rolling uphill. This is one way that the student
might try to explain the ball’s slowing down. This conclusion has to do with the
student’s perception of the experimental environment. It basically claims that the
student misapprehended the levelness of the ground, that what the student at first
thought was level was actually a slight uphill slope. And this is why the ball slowed
down.
The issue with this conclusion,
though, is that we are told that the ground the ball rolls on really is level; the
student hasn’t misperceived that flatness. Note also that the ball doesn’t
need to be rolling uphill in order to slow down. We’ve all witnessed examples of
just what the student is seeing, a ball or other round object slowing down while
moving along a level surface. So, concerning option (B), we won’t
choose this one either because the ground really is flat. And even on flat ground, a ball
will slow down as it rolls.
Option (C) says the student can
correctly conclude that the ball is pushed or pulled by something that the student
cannot see. If this was so, then it would mean
that Newton’s first law of motion still correctly describes this ball’s motion even
as it slows down. The question is, is there something
that could push or pull the ball that the student cannot see?
Well, there is. We think of the frictional force
between the ball and the ground. This force is invisible on this
scale to the human eye. But we know that, nonetheless, it’s
real and it has an effect on the ball’s motion. Notice that this conclusion in
answer option (C) first accounts for what we’re observing in our experiment. And second, it upholds Newton’s
first law of motion. It says that this first law is
still governing the motion of the ball.
Knowing that friction could be the
thing that pushes this ball and causes it to slow down, we’ll put a check mark by
conclusion (C). Before we identify that as our
final answer, let’s consider option (D). This says that the student can
conclude correctly that Earth’s motion around itself opposes the ball’s motion. In this option, if we imagine the
Earth spinning around its own axis, the claim is that this motion opposes the motion
of the ball. So, with the Earth spinning one
way, we imagine that the ball is moving the opposite direction on Earth’s
surface.
But there are issues with this
interpretation. First, we have no reason to believe
that the ball actually is moving on Earth’s surface opposite the direction that
Earth’s surface is rotating. As far as we know, it could be
moving in any direction, including in the direction that Earth rotates. But more importantly, even before
the student gave the ball a push, the ball was already moving in sync with the
Earth. That is, if we imagine the ball
occupying a small patch of Earth’s surface, that patch and the ball on it was moving
with the Earth.
This means that whatever direction
on Earth’s surface the ball was pushed, the effects of Earth’s motion on the ball’s
motion had already been taken into account. So, the movement of the Earth can’t
be used to explain any motion of the ball after it had been pushed. Instead, we need to look for some
push or some pull in the local environment of the ball. So, we’ll cross off option (D) as
well as a conclusion the student could correctly reach. And this leaves us with the answer
we identified earlier, that the student can correctly conclude that the ball is
pushed or pulled by something the student cannot see.
Let’s summarize now what we’ve
learned about drawing conclusions. In this lesson, we saw that a
conclusion is a statement that can be accepted as correct based on the results of an
experimental test of a hypothesis. A hypothesis, we can recall, is a
prediction about some physical phenomenon. And an experimental test is a
carefully designed evaluation of that prediction. We saw further that in order for a
conclusion to be well founded, it must be based on what was hypothesized and
tested. And finally, as we saw in our
example exercise, careful analysis of statements describing experiments is required
for drawing conclusions that are correct.
