In the last experiment, we measured the speed of sound. We did
that by using a high-speed clock to measure short time intervals.
In this experiment, we are going to do things just a little bit
differently. Now that we know the speed of sound, we are going to
use that value to measure short time intervals. From the instant
that I prick this balloon with this needle to the time that the
balloon is completely popped, you could say that we are measuring
the lifetime of a popped balloon.
The experiment will use some of the same equipment from the last
one. A sound trigger is connected to this flash unit. We have two
meter sticks to measure distance. To begin the experiment, I'll
be putting the balloon right here at the 0 mark. It won't take very
long for the sound to travel from the point where I prick the balloon
to the sound trigger. In that amount of time, the balloon won't
rip very far;, it will pretty much be an intact balloon. But then
I'm going to move the balloon further and further down the meter
stick, and the further I move it, keeping the sound trigger in place,
the longer it will take sound to travel from the balloon to the
trigger, so more time will elapse.
About how much time are we talking about? From the last experiment,
you probably found the speed of sound to be about 340 m/s. This
is equal to 340 mm/ms. Now it's useful to think about it in those
units because of the fact that we are dealing with short time intervals
that are on the order of milliseconds rather than seconds. For example,
if I move the balloon from the 0 mark to about 1/3 down the meter
stick. That's about 33 cm, or 330 mm. If the speed of sound is about
340 mm/ms, the amount of time for sound to travel is about 1 ms.
So if, for example, the balloon was 2/3 down the way of the meter
stick, and the popped balloon was pretty much gone at the point,
we can say that took about 2 ms, because each third of the meter
stick is about 1 ms. We can use this to figure out the lifetime
of a balloon.
So let's begin, we will do a series of four bursts. One at 0 m,
one at 0.5 m, one at 1m, and one at 1.5 m. We will see what we get
from those. Here it goes. So in the free frame, we see the balloon
only split a little in the front of it. That's because the amount
of time it took sound to travel from the point of the rip to the
sound trigger was very short. Next I'll move the balloon half a
meter down the stack. I'll point my flash down a little bit, keep
my sound trigger in the same place. Ok let's try this one. Well
that particular shot the balloon was completely ripped down the
front and ripping on the back; the balloon was about half gone.
Let's keep moving down the meter stick. Well that time the balloon
was almost gone, but it still had a little bit of shape of balloon,
so let's move down another half meter. Well in that last shot, the
balloon was almost completely gone. So we can say the lifetime of
a bursting balloon, at least the balloons I am using here, is about
the amount of time it takes to travel 1.5 meters. So using the speed
of sound, and that distance, you can calculate the amount of time
it takes a balloon to burst.