h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

At least I think it's the laws of motion. This one was posed by my 7 year old son, and you have to ignore the obvious *biological* consequences (ie, death) of the scenario he poses.

"If I was standing upstairs, and I fired a rocket launcher downwards at my feet, would I fall down through the hole, or would I travel upwards because of the rocket launcher pushing me away?"

Well, *I* understood the question, and I thought it a reasonably intelligent one. But I don't know what the answer is. I suppose it's gravity versus the equal and opposite reaction of the force of the rocket launcher. Any physicists able to provide him with an answer?

Many thanks.

Mol

I'm not an expert on weapons, but I believe the rocket launched by a rocket launcher is self-propelled, so doesn't have much kickback, so that wouldn't send your son flying upwards. A rocket exploding at his feet probably would though.

Well, he's basing his experience of rocket launchers on PS2 games, and seems to think they have quite a kickback (or whatever the technical term is). But perhaps it is the rocket rather than the launcher. I only play SingStar so I don't really know

Mol

Okay let's assume it does have a recoil (remembered the word). That will be pushing upwards while the force of gravity on your son (and on the weapon he's holding) will be pulling downwards. Whichever is stronger determines which way he heads, but even if he's thrown upwards he'll come straight back down again.

Ooh, good point. So his likely motion is upwards, *then* downwards? Two for the price of one; he *will* be excited. Thank you.

Recoil, that was it. I kept thinking ricochet which I knew was something completely different.

Mol

Recoil is that "peyowwww" noise that bullets used to make.

I meant ricochet.

My husband has just returned from the pub and explained to me that a rocket launcher is simply a tube to enable you to give direction to the rocket.

Anyway, he would like to carry out an experiment this weekend to look at this whole question. Obviously we would like to avoid the biological consequences (ie, death) of a total renactment, and I would quite like to avoid making large holes in the house. And we don't actually have any rockets or launchers.

Any ideas ?

Mol

yes, I'd say your best chance would be with Russian army officers.
The Russian army has still quite some stocks from the soviet times, and there'll be lots of officers willing to sell you the equipment you need for your experiments... probably for quite a reasonable price as well.

ricochet





jf

Kit for an outdoor harmless(?) rocket launcher:

1. Length of plastic drainpipe (approx. 1m)
2. Plastic bottle 3/4 full of cola (must fit inside the drainpipe)
3. Mentos



t.

http://www.youtube.com/watch?v=CD2X0juwHkU



t.

I'm going to suggest that turvy's suggestion won't lift a 7-year-old into the air, but is likely to leave him very sticky.

We were thinking more of a scaled down experiment to lift something like Action Man up in the air, and then back down through the hole blasted into ... something.

Mol