Science Buddies: "Ask an Expert"

My son's science fair project is based on an air rocket toy (you jump on a bladder that is connected by tube to a pipe on which a closed end rocket is inserted). He noticed that the distance the rocket is inserted on the pipe has an effect on how high the rocket goes (or how long the rocket is in the air since we can't measure how high it is actually going). For example, if you insert the rocket so it only covers 2 inches of pipe, it will be in the air for 1 second. If you insert the rocket so it covers 4 inches of pipe, it will be in the air for 3 seconds. Our theory (does this sound right?? someone validate): The forces acting down on the rocket are friction (rocket slip fit over pipe), gravity, and the forces acting up on the rocket is the air as it is expelled from the bladder. What's making the rocket go higher (to a point) when more of it is inserted on the pipe: Does it go higher b/c there is less air leakage (b/c better seal between rocket and pipe) or b/c of less volume (therefore higher pressure when the bladder is jumped on). Our problem: he's in 2nd grade and we are having a hard time deciding whether or not he should understand this (are there any kid sites to help explain 'force' in kid terms) or if we could limit the project conclusion to something he can grasp (and what would that be). I guess I'm looking for feedback on what is considered an age appropriate conclusion for this experiment (or help in how to explain it to him).

Dear Nicho's Mom,

Congratulations!!! Your son is a true scientist... since he has noticed something "odd", which is the first step in scientific discovery.

I'm going to make some assumptions about your set-up from the description, please check these assumptions for validity:

1) Nothing holds the rocket in place while your son jumps on the air bladder, save the friction of the "Rocket" on the lauch tube?

2) The "rocket" does not have a pressure holding vessel or bladder, but is a simple tube, open on one end and closed on the other?

3) Your son has a toy known as a "stomp rocket"?

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If so, this is not in fact a rocket, but an inside out bullet and gun. Just as a long barrelled gun can, but not always, mean a faster muzzle velocity than a short barrel, the "rocket", which inserted over the pipe will have a longer time to be accerated by the high pressure in the bladder and tube when inserted over more length of pipe, before the pressure is released as the "rocket" separates from the tube. If the rocket is inserted on the tube by a very short length, hardly any time, and thus very little accerlation, occurs before the pressure is released and the "rocket" is on its own.

A real rocket would covert a high pressure, but slow moving gas inside of a reaction chamber (the source of the high pressure gas) to a low pressure, but very high velocity gas exiting the rear of the rocket motor. This is the action which creates the Newtonian reaction to propel the rocket.

In your son's "rocket", no such action/reaction takes place between the rocket and exiting gas. Instead the high pressure gas inside of the bladder and tube transfer energy and momentum as the gas expands inside of the tube as the rocket slides off of the tube. As soon as the rocket has left the tube, the pressure is no longer available to further accelerate the rocket, save for a tiny residual that might be inside of the "rocket".

Think of this as being like the piston in a cylinder inside of an engine, save that this piston goes flying !!!

Personally, I think a seven year old can and should be able to grasp the principles of operation of this device.

If you would like your son to experiment with a real rocket, my advice would be a "water rocket". For less than $10, you can't go wrong !

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Hi nichos mom,

Not a great deal to add to Candice's excellent explanation. I think she hit the proverbial nail on the head! The most likely explanation is the length of time that the 'rocket' is attached to the pipe - the more "pipe time", the higher the internal pressure, which consequently gives a greater height / time of flight. This "pipe time" is directly related to how much pipe is covered. As you and your son observed, the greater the length of pipe used = greater flight time, and vice versa.

The only thing I would add is that since this is a "stomp rocket", the amount of force applied to the air bladder will also have an effect on the internal pressure in the projectile. In order to minimize any variation - and therefore produce more accurate and repeatable results - I would recommend using a weight of some description. This can be dropped from the same height each time and therefore produce the same force for each test. As with all science experiments, the best (only?) way to study something is to keep all variables the same, except the one you are trying to study. In this case, pipe length and the resulting flight time.

Please be careful to watch your toes, and let us know how it goes!

Best wishes,
Geoff.