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Re: [Phys-l] Turning effects of a moving bike



This issue has had me thinking for many years about whether the condition
for rotational equilibrium is a consequence of Newton's Laws or is
something
imposed in addition to them -- what do the rest of you think?

Steve Highland

Net Torque=dL/dt, applied to a system, using any inertial origin OR the
system CM as origin, is provable from Newton's laws alone. If one adds the
constraint that all internal forces are central forces, then the internal
torques drop out in pairs and Net External Torque=dL/dt. All of this ( and
the use of non CM or non inertial origins) is standard matter in most
intermediate Classical Mechanics texts.

Bob Sciamanda

I guess what's bothered me over the years is why it is legal to take F=ma
and basically do "r x 'F=ma'" to create the rotational analog for any choice
of r. Is this purely mathematical, or is there some physics in it, too?

What reasoning assigns each force vector (and the ma vector as well) a
particular location in space? What is it that tells us where they act?

Steve Highland