Let a uniform rod of mass
and length
is rotating with angular velocity
about an point
which is at the position of
from one end of rod .
At time zero, the rod is placed gently on a rough, level surface having coefficient of friction
.
If the total time taken by the rod to stop completely , then for different values of position , the time taken by the rod in various experiments are different.
Suppose the maximum possible time taken for the rod to slow is and the minimum possible time is , find the value of :
.
Details
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The torque as a function of P is
τ ( P ) = L M g μ ( ∫ 0 P x d x + ∫ 0 L − P x d x ) = 2 1 M g μ ( L − 2 P + 2 L P 2 )
The rotational inertia as a function of P is
I ( P ) = L M ∫ − P L − P x 2 d x = 3 M ( L 2 + 3 P 2 − 3 L P )
The angular acceleration as a function of P is
α ( P ) = I ( P ) τ ( P ) = 2 ( L 2 + 3 P 2 − 3 L P ) 3 g μ ( L − 2 P + 2 P 2 / L )
Differentiating the acceleration with respect to P and setting it equal to zero (to find a minimum or maximum) yields P = 2 L which implies that the other (either min or max) is at an endpoint. Plugging in the values you find:
α m a x = 1 . 5 ..... which means ..... T m i n = 1 . 5 ω
α m i n = 0 . 7 5 ..... which means ..... T m a x = 0 . 7 5 ω
And thus T m a x T m i n = 0 . 5