Step 6: Speed testing

Since I earlier made rash claims about shooting ping pong balls at supersonic speed, I need to describe how these speeds were measured.

I made a "ballistic pendulum"; a device once used to measure  muzzle velocity of rifle bullets, but now mainly used in physics classes to demonstrate the principles of conservation of momentum and energy. The projectile is shot into a block, and is retained inside the block. The block + projectile then swing due to the momentum of the projectile. The vertical height gain of the block is measured, and when multiplied by the mass of the block + projectile, represents potential energy increase. Using the following equation gives the velocity of the projectile:

Velocity = (m+M)/m (√2gh)

m = mass of projectile, in kilograms (ping pong ball mass = 2.4 g = 0.0024 kg)
M+m = mass of projectile, in kilograms (1.32 kg)
g = acceleration of gravity, 9.81
h = vertical height gain, meters, less height gain from air only tests. (2.5 cm = 0.025 m)

The numbers in parentheses are the average values from my tests.

“A ballistic pendulum is a device for measuring a [projectile's] momentum, from which it is possible to calculate the velocity and kinetic energy. Ballistic pendulums have been largely rendered obsolete by modern chronographs, which allow direct measurement of the projectile velocity. Although the ballistic pendulum is considered obsolete, it remained in use for a significant length of time and led to great advances in the science of ballistics. The ballistic pendulum is still found in physics classrooms today, because of its simplicity and usefulness in demonstrating properties of momentum and energy.” (From Wikipedia).

Photos below show the ballistic pendulum I made (along with a view of my screwdrivers and chisels).
The lines on the panel in the background are to measure height, in centimeters.
The laser pointer shows height of the block. In the photos, the first shows height before firing, the second shows maximum height just after firing. A video camera records the action.
The last photo shows the ping pong ball caught in the block.

In addition to momentum from the ball, there is momentum transferred to the block from the air exiting the cannon. I made some test runs using air only and subtracted this average height (0.03 cm) from the tests with ping pong balls.

<p>Does the ball break if it is fired in an open field? Will this work with a household vacuum? Answer soon please!!</p>
<p>Hey! Please help me before this Sunday! I need to make this for a project due Monday. I just have a couple questions. How did you make the square thingies stay on the PVC? and also, is the vacuum sucking air? or blowing air? and also, do you remove the square pieces? I do not really understand this.</p>
How much distance can you get with this cannon?
<p>It depends on the projectile. If you shoot a ping pong ball, you can't go as far as you would be able to if you shot a more aerodynamic object.</p>
<p>My AP Physics class did a lab on this for projectile motion. We made our own projectiles out of anything from markers to batteries and shot them out the window with the cannon. The most impressive projectile traveled over 100 meters in a horizontal direction, and everyone in the hallway and around the classroom was interested in the cannon :) if you ever get the chance to do this, jump at it!</p>
<p>I really want to try this. Then I want to scale it up and use a tennis ball.</p>
mythbusters made an episode about this... maybe you should watch it and get some ideas about your upscaling effort... post the instructable please
Thanks for the comment. <br>Interesting idea, scaling up. You will need a pvc pipe with ID of about 2.6&quot;, since a tennis ball is 2.57&quot; OD. I don't think there standard size piles of that dimension, so it may be a challenge.<br>Let me know if I can help.
<p>I made this with a 10ft PVC section and I found that Wax Paper seemed to work the best for me. </p>
<p>Thanks; next time I will try waxed paper.</p>
If you used an object that had little to no air flow resistance such as a dart shaped object that is mirrored could you achieve higher speeds? If you test this could you let me know because this could be an amazing project.
<p>I have set the vacuum cannon aside for a while, may do more testing later, will let you know. There is no air resistance to ANY object in the vacuum cannon since the object travels in a vacuum.</p>
<p>Nice toy and video! I'm wondering how much of the damage, could possibly be attributed, to the jet of air pushing the ball? Have you tried to shoot something, without the ball inside? It still would have to pack a punch, it's hard to imagine, the lightweight ball, doing nearly that much damage. But, compressed air, at supersonic speeds...seems like it wouldn't have any trouble, doing that, at that range. </p>
<p>Dico your theory is intriguing but I have to mention a basic equation in physics that explains that your statement is not true. F=m*a in which F=force, m=mass, and a=acceleration. Finally you may say that the air is near the same mass as the ball but a property of fluids (in this case the air) is that it will follow the surface of any cylindrical object. I hope this informs you that the damage done is indeed done by the ball. </p>
<p>Not to pick, but above the speed of sound air is basically not compressible. His Q is a good one&mdash;and pretty easy to test&mdash;just fire the thing empty? Or with a cotton ball? Or....</p>
<p>Doug, thanks for pointing out that above the speed of sound air is basically not compressible.</p><p>I never did confirm that the ball actually travels at supersonic speed, but have found a device to measure it.</p><p>Bill</p>
You should use clear duck tape on each end.
<p style="margin-left: 20.0px;">What happened to your excellent video ?</p><p style="margin-left: 20.0px;">I wanted to show my wife you did this before Mythbusters.</p><p style="margin-left: 20.0px;">A</p>
<p>I may have fixed it now, kind of forgot how to embed a video. </p>
this is so cool but i cant help but think how much better it would be if you replace the near side of it with a barrel sealing piston valve and a high pressure tank because theres the full force of upto 150 psi of pressure pushing a ball against no resistance. the ultimate spud gun.you could load potatoes into it and put them on the moon. <br>intergalactic potato launcher
Tanks for the comment, Monty. <br> <br>Yep, I already have th materials for what you are thinking about. There will be pressure on one side of the near seal, vacuum on the other.
glad you like my idea. if you decide to do it, could you post or update an ible and also is there some way you could make the far seal permanent like a piece of rubber on a hinge? <br>Great ible and idea by the way
Thanks Monty.<br><br>I will make a new Instructable, but I will first find another way to measure speed. <br><br>I also used just flat smooth discs cut from food containers, or soap containers. The disc can just be placed in front of the pipe, with the pimp running the vacuum will suck the disc tight .
try measuring the speed by having 2 lasers pointing a 2 ldr's or other light sensors and have them 50 cm apart. then make some circuitry so when one beam is broken a timer starts and when the other one is broken it stops. then just S=D/T. <br>
you could even wire the ldr up to a mic input of some device and record audio and use an app like Audacity to check the speed
Thanks guys, but electronics is not my strong suit. I understand the concepts, but you will have to tell me what a &quot;ldr&quot; is.
An ldr ( light dependent resistor ) is a simple device whose resistance changes depending on the amount of light it receives and can thus be used to detect a passing object if used in conjunction with a laser.
Thanks Monty -<br><br>I will work on this. Do you hav a suggestion for a timer?
I would either use a standard stopwatch and solder new wires to the buttons or if you have any programming experience then use an arduino.
Interesting project, but as you mentioned, it being supersonic is questionable. I'm used to firearms, and all supersonic rounds give off an audible and distinguishable crack as they break the sound barrier. This didn't. It might be close, but I don't think it made it. <br> <br>Cool project though, straightforward 'ible. All things considered, nice work.
The audible &quot;crack&quot; is from a wall of sound waves building up in front of the projectile. As there is no air forward of the projectile until it exits the tube, and is only outside the tube for a few centimeters in the video, any supersonic crack would be very small. <br> <br>But I'm also having a hard time understanding how the air behind the projectile can expand at a rate faster than the speed of sound when it's at (essentially) STP until the membrane is pierced. <br> <br>I really wish you had a friend with a reliable chronograph.
I don't think you have a great understanding of how a sonic boom works. In order for an object to be supersonic, it must be faster than those waves. For the pressure waves to be in front of an object, as you say, means that it is not supersonic. The object must lead the waves in motion, not the other way around. Also, those waves propagate behind the object and spread outward. Seems to me it could very well affect the atmosphere behind it and create a boom throughout its launch. Mind you, I'm not entirely certain if it requires atmosphere in front as well as behind, or only behind where the waves propagate. But I would point out that if the boom was as short as you say, you'd still be able to hear it. They are incredibly recognizable. The sound would be heard over others going on, and would reverberate off the walls at least allowing you to hear something of it. It'd be comparable to firing a suppressed gun with ammo that breaks the sound barrier at something a few inches away. The suppressor muffles only the noise of the expanding gasses, but does nothing to the sonic boom almost regardless of distance to target. One exception being firing into a solid with the barrel pressed against it.
the sound waves generated by an object radiate from it all around. As the velocity of that object increases, the sound waves in front of it are compressed as the waves catch up to one another until they form a front of extreme pressure at the leading edge of the object generating the sound. The &quot;boom&quot; is generated when it breaks through that pressure wave and surpasses the sound waves it is generating. <br>I'm with cchubb in thinking that the vacuum in front of the projectile would negate any sound waves from being generated. With no sound waves and no air, there would be no high pressure wave front to break through. The air entering the cannon can move as fast as 1640 fps. Sound travels at 1129 fps. In it's vacuum environment, I think the projectile will definitely be moving faster than sound, given it's low mass and speed of acceleration, but without any air pressure in front of it and thus no building sound wave front, there would be no &quot;boom&quot; as it surpasses the speed of sound. Once it hits the atmosphere, there isn't going to be enough inertial energy from the ping pong ball to displace that volume of air at a supersonic speed. I'm wondering if the can wasn't there, would the ball break apart on it's own when it hit the atmosphere?
You're incorrect in assuming that its always traveling through a vacuum. Rewatch the video, there is a space between the end of the barrel and the soda can that is not in vacuum, and in fact is open to the atmosphere. Based on that, your question regarding breaking apart is irrelevant. It hits the atmosphere before it hits the can, and the damage on the ball was done by the can. <br><br>Also, you're incorrect in stating that sound travels at 1129 fps. The speed of sound changes with temperature. You picked a value for a very specific temperature, which could be used to guess an altitude. Keep in mind that the speed of sound is not a constant value. for instance, people say Felix Baumgartner broke the speed of sound, when in reality he broke several speeds of sound.<br><br>Another thing, &quot;inertial energy&quot; doesn't exist. There is inertia, and there is energy, there is no &quot;intertial energy&quot;. There is however, kinetic energy, which is what I think you're trying to refer to.<br><br>It is evident to me that you do not understand what you're trying to talk about with all of the errors you have made. Consider that the energy you're trying to refer to exists for all moving objects, and that it increases as velocity increases. Consider also that anything that moves through the air displaces the gas. Now, if something with as much energy as a ping pong ball moving at supersonic speeds (at whichever temperature you choose), cannot displace the air in front of it, then how does a ping pong ball tossed through the air displace the same volume? If what you said were true, then playing ping pong would not be possible, because the ball could not physically move through the air, and would act like it hit a solid the instant you released it.
Thanks, Baratacus - <br> <br>You said &quot;The air entering the cannon can move as fast as 1640 fps.&quot; <br>How do you get that number? (Just curious). <br> <br>No, the ball did not break up unless it hit the can or another object. However, I have wondered if it might heat up to its melting point if it was moving fast enough. All the kinetic energy would be converted to heat. <br> <br>
Granted, I'm no expert at supersonic waves, but I have fired suppressed guns with projectiles both above and below the speed of sound. The &quot;boom&quot; from a supersonic round from a suppressed gun is loud, but I have never fired it at a target inches from the muzzle to determine if the sonic boom requires a significant stretch of non-turbulent fluid to pass through. <br><br>In the area within inches of the muzzle it's going to be very turbulent during and after the projectile leaves the barrel. I think that sound of a supersonic projectile is going to be very hard to detect over the sound of the membrane breaking, the can rupturing and other noises. <br><br>A chronograph is going to tell the whole story. Ideally one within the last couple of inches of the barrel, but one directly outside might have some bearing if the projectile is heavier than a ping pong ball, and more aerodynamic, like a golf ball, though attendant safety measures will be much more stringent with such a heavy projectile.
I'm uncertain how much turbulence has to do with it. If a fighter flies through fairly turbulent storms at supersonic speeds, it'd still result in a boom. Though this is far different from that. I agree with you, a better measure is necessary to determine this cannons projectile speed.<br><br>Also, I must applaud you. It's rare that I find someone willing to have a pleasant discussion without resorting to personal attacks anywhere on the Internet. I feel you and I would get along quite well.
PS: Will you come organize my workbench?
Thanks for the comment. I'm still trying to understand this thing. I am working on an upgrade that could double whatever speed it gets now.
To Prophes0r : I believe one part of your post may be flawed: <br> <br>You state: <br>&quot; Both will have to contend with the friction of the ball against the barrel.&quot; &amp; &quot;Both cannons also experience some loss as the ball smacks against the sides of the barrel since they are of differing diameters. &quot; <br> <br>If the projectile is round (as is the case with a ping pong ball) and is slightly smaller in diameter than the barrel, the ball will not touch the inner surface of the barrel because the air behind the ball is equally distributed on the surface of the ball. As the gun is fired, the pressure behind the ball will try to escape around the ball equally, causing a small 'cushion' between the ball and the barrel. This principle is the reason that the big guns on &quot;Pumpkin Chunkin&quot; can actually shoot the pumpkin projectile without obliterating the pumpkin. If there is a problem (probably due to the imperfect shape of the pumpkin or too much pressure), when fired, if part of the pumpkin touches the barrel, it &quot;pies&quot; as they call it, which just means the pumpkin is obliterated. With a cushion of air around the pumpkin, it can move through the barrel with no friction. You do have a loss of pressure which reduces the pumpkin's potential velocity as it travels through and out the barrel, but the pumpkin/barrel friction would be negated. I'd picture a vacuum cannon as having the same quality since the flow of air would still want to escape around the ball. <br> <br>
Tanks, Handy - <br> <br>I believe you are correct about the air 'cushion' between the ball and the barrel. There would be minimal friction between ball and barrel anyway.
I have published a vacuum cannon video and have credited you for the inspiration in the description: http://www.youtube.com/watch?v=CVL99yIB3NQ
Fun ! <br> <br>But this is NOT a workshop&hellip; it's a LAB !!!!&hellip;
You through now Vinnie.
I'm sorry ! <br>I was not aware I messed up that bad. <br>I really didn't mean to send more than one message &hellip;&nbsp;I simply thought it didn't get through as my screen froze. <br>I'm really sorry.
Merci, Vincent. <br> <br>I also send multiple e-mails, but you set a record :) <br> <br>I can not decide if I want to be a woodworker or a science experimenter in my retirement.
Very cool. Thank you. High speed cameras in the 1,000 fps category are in the &quot;I can afford that&quot; range now, but in order to use it you'll need to expand the view into the messy end of your shop :)

About This Instructable




Bio: I'm a retired mechanical engineer, woodworker, boater, and inventor. Now I'm getting into wood turning, and have found that all my wood projects ... More »
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