April 06, 2011

Mythbusters need to spin more science

I want to like this segment that the Mythbusters did on the merry-go-round spun by the bullet.  I think it's an interesting question based off of a scene from a movie: Is there enough kinetic energy in a bullet to spin a merry-go-round.

A proper analysis of the question could involve kinetic energy, rotational kinetic energy, torque, angular speed, angular acceleration, angular momentum, friction, air resistance and potentially other factors.

The Mythbusters only vaguely waved their hands at some of these ideas and presented physical parameters in weird units.

The voice over in the above video said "First, the science of spin..." then proceeded to say very little about rotational motion.

Grant talks about how the bullet has to "overcome inertia".  Okay....I mean, we tell physics students that in rotational motion it is the moment of inertia that matters, but I'm willing to let it slide. EXCEPT, he then pulls out a force gauge and measures how much force is needed to just get the merry-go-round to move. But, since we care about the rotation of the merry-go-round, it's the torque that matters.  Plus, what he really is getting at is a measure of the coefficient of static (rotational) friction on the axle.

There were a lot of other problems with the things the Mythbusters said in the segment.  Some of what they said was just using colloquial terms for physical properties which a physicists would not use.  I'm not so terribly upset by those comments.  I'm just bothered by the lack of coherent explanation of the physics of the system.  I came up with a simple analysis of the experiment, using parameters provided in the video and considering just conservation of momentum. I don't claim it to be a full treatment of the question, just more of a back of the envelope type calculation.  (And I don't take friction into account, either.)

I turned my calculation into a pencast:

Analysis of Mythbusters merry-go-round question

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