Newton's Second Law
for Rotation about a Fixed Axis
Just because it moves in a circle, an object of mass m must have a centeral force on it -- a radial force, a force toward the center, or a centripetal force.
In addition, consider an object of mass m that has a tangential force on it Ft,
This means the mass m will have a tangential acceleration, at
Ft = m at For a moment, think of this as a mass m embedded in a massless disk -- like a thin sheet of plexiglass or a thin piece of aluminum. How can we relate this tangential accleration of the small mass m to the angular accelration of the entire "system" or disk?
We know
at = r The torque due to a tangential force is clearly
= r Ft = r ( m at )
= r m ( r )
= [ m r 2 ]
= I
where, again,
I = m r2 is the moment of intertia, the rotational equivalent of the mass, indeed, the "rotational mass".
Notice, of course, that this rotational version of Newton's Second Law,
= I looks very much like the linear form
F = m a with which we are already quite familiar.
This immediate example had only a single mass m. If more masses are involved then, just as before, the moment of inertia is
I = [ mi ri2] and, for an extended or continuous object, this becomes
r is still the perpendicular distance from the axis of rotation.
Typically the density will be a constant so we will have
Once the moment of inertia is found or given, we are back to applying Newton's Second Law -- this time for rotation,
= I
Examples Torque Work, Power, and Energy Return to ToC, Ch10, Rotation about a Fixed Axis (c) Doug Davis, 2001; all rights reserved