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Balloon "Jet" Propulsion
Name: Lisa
Status: Student
Grade: 6-8
Location: IN
Country: United States
Date: January 2009
Question:
How does the size of the balloon neck affect the distance it
goes when the air is released?
Replies:
Hi Lisa,
A smaller neck releases less air every second, but because of this, it
takes longer before the balloon runs out of air (hence longer before
it runs out of propulsion). However, the energy stored is the same in
either case, since the neck size makes no difference to the volume of
air stored under pressure in the balloon.
So logically, since the energy stored remains the same (regardless of
neck size), the distance travelled should be the same. Alternatively,
you can think of it this way: A smaller neck produces less thrust, but
that thrust will last longer (since it takes longer for all the air to
escape from a smaller neck). So a balloon with a smaller neck will
accelerate more slowly, but because it will be able to accelerate for
a longer period, it should reach the same distance as a balloon with a
larger neck that accelerates faster, but "runs out of gas" sooner.
Regards,
Bob Wilson
It probably does have some effect, Lisa,
but I'm not sure whether the effect is strong or weak, or which neck-size
is best.
We could presume the charge of air inside is good for a certain number of
Newton-seconds of impulse.
For example, 2 Newton-seconds of impulse could be
a) 0.5 Newtons for 4 seconds with a narrow neck, or
b) 2 Newtons for 1 second with a wide neck.
It is plausible that a certain impulse will push the balloon a certain distance.
So maybe size does not matter.
But then, a balloon has more air-drag when it is bigger.
Not sure it helps us to know that.
The balloon is always going to be big for some time, then smaller for
some time.
The ratio of the two times is probably the same for every nozzle-size.
And drag goes as the square of velocity,
so going faster by having a wider neck might be wasteful.
I think that will matter noticeably. Smaller may be better.
And jet nozzles are often less efficient when too small.
How small that means in this case, I am not sure.
Ratio of width to length comes to mind.
I.E., length greater than twice the width sounds bad to me.
Of course size means width, diameter, mouth area etc.
of the neck, not its length.
A longer neck probably just wastes energy by slowing down the air streaming out..
A long neck causes that spluttering noise a balloon sometimes makes.
That is a very inefficient nozzle.
How these factors all weigh together, who knows?
Therefore it s a good subject for an experiment,
if someone has a convenient way to make nozzles of differing width.
(And a frictionless way to make the balloon go straight...)
If you can find some kind of lightweight tapered plastic cones
and snip off differing amounts of the narrow tip,
and jam the wide end into a fat balloon neck.
I suspect that would be perfect.
The nozzle efficiency would be very good,
and the effective length/width ratio would be small,
and it would be the same for all opening diameters you create.
Soda straws can probably be heat-softened for a couple inches in the middle
and pulled lengthwise to become narrow in the middle.
After you cut it at the narrowest point, that would make two nozzles.
Jim Swenson
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Update: June 2012
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