A while back on MB Jamie stated that “Driving two cars head-on into each other at 50mph damages each car the same as driving one car into a wall at 100mph”

Controversy ensued.

Of course they had to actually try it, and found that the 100mph into a wall car got twice as squished.

Is this more or less the same problem as our canon?

Turns out karlschneider had the clue I needed. I’ve never seen such devotion to a single piece of music in my life as the entire wiki-like website devoted to the song Popcorn: http://www.popcorn-song.com/ With the help of it and discogs.com I have determined which version of Popcorn is in this video. Yes, majutsushi, Jean Michel Jarre did a version of this song in 1972 but that’s not his version. This is Popcorn (Radio Mix) by M & H Band in 1987, which can be heard on YouTube: http://www.youtube.com/watch?v=yZr36fczU88

The guys at the X-ray machines in airports will be more than happy to learn one more thing to look out for.

Yes, but the cannon is utilizing some of the reaction to move. I completely understand what you mean with the space ship comparison but that’s a bad one as the framework is different. The propulsive force in a rocket is attached to the rocket, in this situation the propulsive force is external to the object being acted upon. If the cannon was immobile, there would only be one direction for the explosive force to go. As it is, the explosive force acts upon more than one object – both the ball and the cannon. Immobilize the cannon and all the energy would go into the ball.

There’s a fixed amount of kinetic energy available from the explosion. Divide it out by the objects moved and you have an idea of the energy going into each. Reduce the number of objects moved and you’ve got more energy going into each. Or am I totally off base here?

The other “leak” is the relatively large fuse hole. He should have tightened that up quite a bit.

FYI, as a kid I had a very small single-shot cap gun that I modified in a similar manner. Its ammo was the head of a finishing nail (which I clipped off with a pair of pliers). Since it was powered by a single cap it wasn’t very strong; just enough to embed the nail head into a piece of cardboard (certainly strong enough to damage somebody’s eye). Luckily I outgrew that phase.

Both the cannon and the cannon ball receive equal and opposite momentum (from conservation of momentum), but this is not the same as equal and opposite KE.

Arggh. Right you are. Momentum, not kinetic energy.

This is what I get for posting while half-asleep.

Thanks for the correction.

The ball continues to accelerate as long as it is in the barrel being pushed by the expanding gas. The gas is expanding against the barrel walls and the projectile.

This means that any force applied to the barrel wall while the projectile is still in it (and therefore still being acted on by the gas)must be added to the inertia of the barrel, the gas and the ball.

Anchoring the canon would be the same as applying equal and opposite force against its thrust.

But because the barrel is so short the period of acceleration would be short and the effect would be tiny.