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1.)

A uniform rod of mass 0.93 kg is 17 m long.

The rod is pivoted about a horizontal, fric-

tionless pin at the end of a thin extension (of

negligible mass) a distance 17 m from the cen-

ter of mass of the rod. Initially the rod makes

an angle of 70 degrees with the horizontal. The rod

is released from rest at an angle of 70 degrees with

the horizontal.

The acceleration of gravity is 9.8 m/s^2.

Hint: The moment of inertia of the rod

about its center-of-mass is 1/12 ML^2.

What is the angular speed of the rod at

the instant the rod is in a horizontal position?

Answer in units of rad/s.

MgL/2 = .5(1/12ML^2)w^2

This works out to be:

w = sqrt(12g/L)

w = 2.630142 ???

But the answer is wrong?!?! why?!

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2.

Two pulley wheels, or respective radii R1 =

0.34 m and R2 = 1.5 m are mounted rigidly

on a common axle and clamped together. The

combined moment of inertia of the two wheels

is I + 4.5 kg*m^2.

Mass m1 = 18 kg is attached to a cord

wrapped around the first wheel, and another

mass m2 = 5.9 kg is attached to another cord

wrapped around the second wheel.

The acceleration of gravity is 9.8 m/s^2 :

Find the angular acceleration of the system.

Take clockwise direction as positive. Answer

in units of rad/s^2.

I = .5MR^2

I1= .5(18)(.34)^2

I2= .5(5.9)(1.5)^2

I = I1 +I2 + 4.5 = 12.1779

Acceleration= [(m2gR2)-(m1gR1)]/[I+(m1r1^2)+(m2r2^2)]

= .971689752 ?!?!

This is wrong as well, WHY!?!?!?

Please help. All help is appreciated.

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# Homework Help: Rotational Motion and Moment of Inertia Questions

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