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Re: [Phys-l] Ballistics divertissement



John Denker wrote
I've had kids as young as 8 ask the key physics question: Why does the
rocket need both air *and* water? I answer that the air is needed for
energy, and the water is needed for momentum. I then explain what that
means.....

I laud you for teaching kids physics (and I really like the way you
explain things to us!) but, unfortunately, kids learn at an early age
that science is about using scientific words. I think it would be
better to explain first and use the words "energy" and "momentum" later,
perhaps when the children are older. For an 8 year old: On earth, when
we want to go forward faster we push back against something--the floor
when we're walking, the road when we're biking, and the water when we're
swimming. A rocket doesn't need anything outside itself to push back
against; it carries stuff to push back against with it. That's why a
rocket works in the vacuum of outer space. The water rocket works
better when it has more stuff than air with it to push back against.
That's why it works better when you put some water in it.