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Transcript by "ResponsibleCitizen64" (Intro music)
Okay there’s a lot of stuff I want to cover in this video so it’s going to run a little
long. I'm going to break it up into 3 main sections: history of chronograph technology,
the theory behind the pendulum chronograph, then a demonstration of a pendulum chronograph.
Ever since the invention of gunpowder and the field of ballistics as we know it, one
of the main questions has been how fast does the projectile move? This is usually expressed
in the terms of meters per second or feet per second. In an earlier video I showed you
how to use a modern digital chronograph to measure bullet speed. But what did they do
before they had modern technology? One early chronograph used two metal screens to trigger
a mechanical timer. Before that they used two spinning discs on
the same spindle. The discs would spin at a high rate of speed, synchronized to each
other. The bullet would be shot through both discs. The bullet would impact the first disc
it would leave a hole, then impact the second disc. But the two holes would be at different
angles because the discs were spinning in between as the bullet passed from the first
disc to the second disc. By measuring the difference in angle they could determine the
time it took for the bullet to pass through. Going back even further then that, to the
mid 1700's, an English mathematician named Benjamin Robins invented the ballistic pendulum.
This was the first time anybody had figured out a way to reliably measure the speed of
a projectile. It started the field of ballistic study as we know it today.
The concept is to have a heavy pendulum that can capture the bullet when a bullet is fired
into it. It relies on the conservation of momentum between the bullet and the pendulum
system. The momentum is transferred from the speeding bullet into the pendulum causing
it to move. By measuring the movement and the energy in the pendulum system, you could
then determine the energy that the bullet had. By knowing the weight of the bullet you
can then calculate what the speed of that bullet was.
Now it takes a lot of very advanced mathematics to calculate all of this. The problem is that
I failed calculus when I was in school. So luckily someone else has already figured this
out. Now, it has been around since the mid 1750's. I was able to get all the calculations
I needed from Wikipedia and there is also a website that has a built-in calculate.
Now real pendulum chronometers are built from heavy iron to handle the high energies generated
by cannons and high powered rifles. For this demonstration I'm just going to use an air-gun
it just makes everything a lot easier. I am using an airgun that is powered by carbon
dioxide. I previously tested this using my digital chronometer and I know it runs anywhere
from 360 feet per second to 400 feet per second depending on the ambient temperature and the
type of pellet I am firing. I'm using a piece of wire as the pendulum
arm and a can of play-doh to capture the pellet. The play-doh is just the right consistency
to capture the pellet completely without letting it blow-through or generating a ricochet.
Okay here is the pendulum setup in front of this grid so I can see how far it moves. I'll
capture the motion with my camera, and then play it back in slow-motion so I can see how
far the maximum movement was. We're ready to fire the airgun so it’s time
Here's the slow motion video, each frame is 1/60th of a second. I could try to calculate
the momentum directly from this motion, but instead all I really need is the maximum height
it reaches. Just for curiosity I wanted to see how far
the pellets went into the modeling clay. They went right to the back without punching through
the back of the cup, just perfect. Now let’s review the data we've collected
so far: I know the maximum height that the pendulum
reached, it’s about 1.9 centimeters. I know the weight of the pellet, it’s about 0.33
grams. And I know the weight of the entire pendulum system, the play-doh and the wire,
it’s about 2.5 ounces. Plugging all of this into the handy online
calculator it gives me a result of 131 meters per second. That's equivalent to 430 feet
per second. Now, earlier I chrono'd this gun with my digital chronometer and it came out
to 400 feet per second, so I'm getting 30 feet per second faster here. I think it’s
probably close enough for this close approximation. Well that wraps-up this demonstration of the
pendulum chronometer. I hope you learned something about physics, momentum transfer, mathematics,
and in general the ingenuity of people; how they did things before we had modern electronics.
It’s always interesting to see the history of certain engineering items to see how ingenious
people were back in the old days. Well, please vote "like" if you like this
video, I hope to do more of these science and technology videos in the future.
(Closing music)