(Music)
what do you do with a bag of potatoes and a potato launcher and what does this
have to do with physics?
well a potato and a launcher are going to show you how
you can determine whether or not there is reliable evidence to support a theory
you have about something. That comes in handy with physics and all science. It's
called the scientific method. It's a methodical way to test a
hypothesis based on data collection and controlled variables. Without it we would
not know that the Earth orbits around the Sun or that nature changes through
evolution or that penicillin kills bacteria. We may not change the world
with the next great scientific principle but let's go for it anyway. Okay, my
physics and motion crew is going to demonstrate how this works. Ready guys?
(Pop)
Nice launch! Now, let's do some launching using the scientific method. First we
need to ask a question. Let's make our question this: does the length of the
barrel affect the distance that the potato travels? Next, let's make a
hypothesis or an educated guess, if I increase the length of the barrel the
potato will travel farther. In the scientific method you can change only
one thing at a time so that you are sure what is causing the
outcome. You call that change in an experiment the independent variable. In
our case it's the length of the barrel. The dependent variable changes in
response to the changing independent variable. For us, that's the distance the
potato travels. But I could launch potatoes all day long and I would only
have a rough estimate of what was happening if I just eyeballed how far
they went. If I really want to nail down what the test shows I have to record the
data and I do that in a table that looks like this. One column shows the length of
the barrel and the other shows how far the potato went. Okay, ready? Here goes the
first one in a barrel measuring 0.3 0-5 meters.
(Pop)
Nice landing! Now, we're going to measure how far it went and time how long it was
in the air.
50 1.8 meters. We're gonna plug that into our table and what was
the time in the air? Three point four seven seconds. Now let's adjust our
barrel that measures 0.45 seven meters and launch another potato.
(Pop)
Another nice one.
Measurement?
Sixty nine point eight meters and how long was that
one in the air? Four point two six seconds. And now for our third length, our
longest barrel, which is zero point six zero one meters. We think that this will
make the potato go farthest, right?
(Pop)
Where did we land? 124 point eight meters and seven point three two seconds.
So, let's see what the table tells us. The longer barrel shot the potato the
farthest. Even though our results may have turned out as expected, we now have
evidence to support our hypothesis by using scientific method. Actually we'd
have to show that we could repeat the experiment and get the same results,
since being able to replicate results is a core principle of science, but for our
purposes this test is enough. Now that we have this data in the table let's graph
it and see what else we learn. By graphing we will see more about how the
lengths of the different barrels effects the potato launch. Graphing the data lets
us see relationships between our variables that we might not see if we
only looked at a table. Graphs also help us make predictions of how the launcher
will behave as you'll see in a minute. Any good graph has to have a clear
descriptive title, let's call ours barrel length versus distance traveled by the
potato. Those are our variables. The independent variable goes on the x-axis
and the dependent variable goes on the y-axis. So what goes on the x-axis? Our
independent variable, the length of the barrel. And what goes on our y-axis? The
dependent variable or the distance traveled. When you do this make sure the
numbers are evenly spaced along the axis. Write out the variables including units
in parentheses, like this, now plot your data points on the graph. Draw what we
call a best fit line, the line that runs through the average of your points. What
does this tell us? The graph supports our hypothesis that the longer the barrel
the farther the potato travels. We can pick a point that wasn't one of the data
points we collected within our graph and see that when the barrel is 0.5 0 meters
long the potato should go 102.3 meters. This method of prediction is called
interpolation, estimating a value within a set of data points.
We can also trace
our best fit line out beyond our data points and predict how far the potato
will go. This is called extrapolation, estimating
a value beyond a set of data points. Let's do another graph this time with
distance on the Y and time on the x axis, then we can talk about slope which tells
us a lot about the relationship between our variables. You can find the slope of
any line in the world and here's how. As you learned in math class a line has an
equation y equals MX plus B, where x and y are your coordinates M is the slope of
the line that runs through the average of your points on the graph and B is
your y intercept, the point where your graph crosses the y axis. To find the
slope of the line, find two points on your line and then find the difference
in the Y values of these points divided by the difference between the x-values
of them or y2 minus y1 divided by x2 minus x.1 Let's look at how long it took
our potato to cover the distance it did, distance in meters on the y axis and
time in seconds on the x axis. If we find the slope on our potato graph by using
the time it took to get to the distance it traveled we will know the speed of
our potatoes. By plugging values into the slope equation we calculate the average
speed of the potato as 10 meters per second. Okay, that's all our potato graphs
can tell us but it's not all that graphs in general can tell us. Let's look at
some other kinds of graphs. This is an inversely proportional graph physicists
use these graphs to compare pressure and volume of gases. When pressure increases
volume decreases or with Newton's law of universal gravitation as the square of
the distance between bodies increases the force between them decreases, F is
proportional to 1 divided by distance squared. It tells us that as one of our
variables increases the other decreases. This graph shows a rapid increase in the
distance covered by a racecar over time which has the form of a quadratic
relationship, Y is proportional to x squared. We have the displacement of the
racecar on the wire and the time on the x-axis.
So we've
collected potato launcher data and graphed it and also looked at other
graphs that help you analyze data and test hypotheses, two very important
factors when you're using the scientific method. Using facts and data is what has
made many of the world's greatest inventions and discoveries possible.
That's it for this segment of physics in motion, we will see you next time.
(Music)
For more practice problems, lab activities, and note taking guides check
out the physics and motion toolkit.