Newtons first law - busted? (in Off-topic)

AdminQBGentlemanLoser [{END}] August 28 2012 5:20 PM EDT

I just watched Mythbusters break Newtons first law, by moving a sailboat that 'blew it's own sail'.

The boat had its own fan, which allowed the sailboat to move under its own power.


Demigod August 28 2012 8:51 PM EDT

Nope. But I can't tell you why. I stumbled on YouTube response videos and comments that try to explain it with varying theories.

Fishead August 28 2012 10:43 PM EDT

Myth Busters science is right up there with the rest of reality TV. I had to stop watching it. There is a series on Yahoo where a guy and a hot girl blow stuff up. It takes the worthwhile entertaining part of Myth Busters and adds a little T&A

Sickone August 28 2012 11:00 PM EDT

Naw, they would have broken the first law only if they were using a coil gun (no exhaust gas) in the back shooting pellets at a collection bucket in front (no mass entering nor leaving the craft), while in a vacuum and without contact to any surface (no other external forces) and THEN gotten net forward movement ;)

As it was, that experiment was basically just using the sail as secondary control surface for the fan (an extremely inefficient one at that too).

The net forces being applied ?
a) Fan sucking in air from the back causing one force, which was roughly equal and of opposite direction to
b) the force caused by the propelled air hitting the sail, but then you also get
c) reflected air going in all directions towards the laterals and back of the sail (mostly lateral, but slightly backwards too), summing up to a much smaller forward-facing supplemental force acting on the sail (since most of its vector components on average are roughly equal and in opposite directions, mostly canceling each-other out).

A and B cancel each-other out, while most of the many forces in C cancel each-other out only partially, leaving you with a smaller forwards-facing force.

Sickone August 28 2012 11:08 PM EDT

P.S. It's pretty much the same as putting a fan inside a tube, and bending the tube 180 degrees.
You'd initially think that since air going in on one end equals air going out the other end you'd be getting no forwards thrust since those two things are canceling each-other out, right ?
But if you do that, you're missing the part where the air flow is being bent, and for that you need a force (exerted by the tube walls on the air), and the reverse of THAT force (exerted by air as it's being redirected on the tube walls) will drive the device forward.
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