[tlb]

You have to look for the equal and opposite reaction. A normal jet creates a stream of fast air behind it. There's no such thing for a reverse jet that sucks air in.

With your concrete block example, as I start pulling the block towards me I experience an impulse forward. But when it approaches my body I slow it down to zero speed, creating an opposite impulse. So although I might have moved forward a few inches during the motion, my momentum is zero at the end. When you scale this up to large numbers of air molecules, the result is the same.

[vpribish]

thanks for following up. I think this is one of those conversations that works better in front of a white board - it's surprisingly tricky to express in words.

if the demo was mainly to show that it's the jet of air expelled out the back that's providing thrust, then, fine it does that and its a valuable lesson. I guess I'm hung up on the technicality that there is actually a real movement of air mass even without that rearward jet and that has to be felt by the apparatus - I guess it's just unnoticeably small in the real world demo.

anyhow, my confidence in physics intuition has been shaken. thanks bunches.