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Re: When Physical Intuition Fails



Michael Edmiston writes:

If the tire does not slip (infinite mu) then all initial rotational
energy stays with the tire, partly as rotational and partly as
translational.

Michael apparently reaches this conclusion by suggesting that "no
slip" means "no dissipation of energy." The problem is that the
conclusion violates Newton's laws.

As Oren Quist mentioned (using different notation), Newton's laws require

r^2
v_f_Newton = w_o*R * ------- (mu > 0)
1 + r^2

where R is the radius of the spinning object, r = R_g/R, and R_g is
the radius of gyration.

On the other hand conservation of energy coupled with the assumption
of no dissipation would require

r
v_f_Energy = w_o*R * ------------- > v_f_Newton
sqrt(1 + r^2)

Something has to give and, in my book it's the "rigid body"
assumption that necessarily accompanies the "no dissipation" concept.

--
John Mallinckrodt mailto:ajm@csupomona.edu
Cal Poly Pomona http://www.csupomona.edu/~ajm

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.