## D-8 ROTATIONAL INERTIA1. PURPOSE:
(1) To demonstrate the application of energy and momentum principles to a rotating system. (2) To measure the moment of inertia of various mass distributions by a dynamical
method and to verify the equation I = mr
Welch rotation apparatus with ball-bearing shaft.
The moment of inertia of a body is the measure of its rotational inertia about an axis. It is determined by the body's mass distribution about that axis. The importance of the moment of inertia lies in its effect on the body's dynamical behavior. In particular, a torque applied to the body will cause an angular acceleration of the body given by
τ = Iα
where τ is the applied torque, I is the body's moment of inertia, and α is its angular acceleration. [Both τ and I must be expressed about the The moment of inertia of a localized or "point" mass about an axis is
I = mr where m is the mass and r is the perpendicular distance measured from the axis to the mass. The moment of inertia of a distributed mass is found by subdividing the body into infinitesimal point masses and integrating over the entire volume of the mass. Most textbooks have tables showing moments of inertia of bodies of simple geometric shape. You will need to consult this table, and you will also need to be able to apply the parallel axis theorem.
The apparatus has a vertical shaft (D) rotating on high quality ball bearings (B). A
horizontal arm attaches to the top of the shaft. Slotted weights (C The rotation is produced in a manner which allows measurement of the acceleration of the system. A string is wrapped around the vertical shaft, run over the pulley (H) to a weight hanger. Weights (W) are placed on the hanger and allowed to fall a measured distance. The time of fall is measured. The falling weight covers a distance y in t seconds, so
y = (1/2)at Use Eq. 3 to calculate the linear acceleration of the falling weight. The net force on the falling body is
mg - T = ma . The net torque on the rotating shaft is therefore
τ = Tr - τ τ The angular and linear accelerations are related by
a = αr . From Eq. 4 through 6 the tension T and the angular acceleration α may be eliminated. The result may be solved for the moment of inertia, I:
I = [(g/a) - 1] m r Eq. 3 may be used to find a, experimentally. Eq. 7 then gives the moment of inertia in terms of measured quantities. The total moment of inertia, I can be written as the sum of the inertia due to the added
weights, MR
I = I In order to investigate Eq. 8 systematically, rearrange it algebraically into a form suitable for graphical analysis. Leaving the details as an algebra exercise, the result is:
MR
A plot of MR The quantity [mr This form of the equation suggests an appropriate data-taking strategy. To obtain a
straight line graph, the quantities in square brackets must be kept constant. All of the quantities
in square brackets
The moment of inertia of the vertical shaft and the horizontal arm can be measured, but cannot easily be directly calculated from geometry. One might be tempted to directly measure their moment of inertia by accelerating the cross arm with no added weights on it. This would require a very small m on the hanger to achieve a time of fall long enough to measure precisely. It would also make the applied force on the shaft so small that the torque of friction might cause systematic error. Another serious concern is whether the torque due to friction is constant for all cases. Friction, as you know, usually depends on the force pressing surfaces together (the loading force.) Here the loading force is the total force on the bearing, which is the total weight of the shaft, cross-arm and any masses on the arm. The only way to hold the friction constant is to keep M and m both constant, allowing R to vary. So you will hold m constant in all cases, for the reasons given above. Then systematically study cases with different values of M and R. It is always a good idea to have a clear strategy in mind for analyzing the data
Your objective is to experimentally verify that the moment of inertia of a mass moving
in a circle of radius R is proportional to the mass and proportional to the square of the radius
of the circle. Therefore it is good strategy to study a sufficient number of cases where M is
held constant and R varies, and still another set of cases where R is held constant and M
varies. You should study the next section, DATA ANALYSIS, In general, the procedure is this: Place an appropriate number of slotted weights in the weight hanger so the acceleration is slow enough to time precisely. Record: h, the distance the hanger "falls" The quantities to be determined are: I Place equal masses at equal radii on either side of the horizontal arm, for good static and dynamic balance. Tighten the wing nuts securely. Consider the meaning of R. It must be measured from the rotation axis to the effective center of the mass. Where is the effective center? It is (1) Take data for various values of the mass M, holding R constant at a value of about 12 cm. Use this data to make a plot of M vs. (1/a). Include the mass of the wing nuts in M. (2) Take data for various values of R with constant M. Make a plot of R
(1) Look at the MR (2) Look at the R (3) You may also plot all of the data on a single graph. Suggestion: Plot R (4) You may also plot all of the data, plotting M vs. (1/a) with R as a parameter. Discusss the significance (if any) of the slopes and intercepts of this graph. (5) τ (6) The equations used in this experiment do not separate the various frictional effects.
The quantity τ
(1) Determine the fraction of mgh which is dissipated by frictional effects. (2) A student assumes that the tension in the string is just equal to the weight hanging from it. (This is, of course, not so.) How much error will this mistake cause (in %) in this student's experimental determination of the moment of inertia? (3) Show the full derivation of equation (7). (4) How large would the systematic error be in the moment of inertia if the mass of the wing nuts had not been included in the calculation? (5*) Derive an equation for the moment of inertia, expressed as a function of the experimentally measured quantities listed in procedure (1). (6*) Derive an equation for the total energy which was lost to frictional heating, in terms of the experimentally measured quantities listed in procedure (1). (7*) Equation (8) was used to calculate moments of inertia. That equation did not explicitly include the amount of frictional energy loss. Yet friction certainly does affect the data—the falling body would accelerate a bit faster if friction were removed. Then why can friction be "ignored" when using equation (8)? (8*) Treating the rotating weights as cylinders, develop an equation to experimentally determine their radius of gyration. Text and line drawing © 1998, 2004 by Donald E. Simanek. |