Variable Friction on horizontal Rod !!

Let a uniform rod of mass M M and length L L is rotating with angular velocity ω \omega about an point P P which is at the position of x x from one end of rod . At time zero, the rod is placed gently on a rough, level surface having coefficient of friction μ \mu .

If the total time taken by the rod to stop completely T T , then for different values of position x x , the time taken by the rod in various experiments are different.

Suppose the maximum possible time taken for the rod to slow is T m a x { T }_{ max } and the minimum possible time is T m i n { T }_{ min } , find the value of :

100 × T m i n T m a x 100\quad \times \quad \cfrac { { T }_{ min } }{ { T }_{ max } } .


Details

  • L L = 10 m
  • M M = 10 kg
  • μ \mu = 0.5
  • g g = 10 m/s^2
  • ω \omega = 10 rad/s
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The answer is 50.

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1 solution

Nathanael Case
Dec 29, 2014

The torque as a function of P is

τ ( P ) = M g μ L ( 0 P x d x + 0 L P x d x ) = 1 2 M g μ ( L 2 P + 2 P 2 L ) \tau(P)=\frac{Mg\mu}{L}(\int_0^P xdx+\int_0^{L-P}xdx)=\frac{1}{2}Mg\mu(L-2P+2\frac{P^2}{L})

The rotational inertia as a function of P is

I ( P ) = M L P L P x 2 d x = M 3 ( L 2 + 3 P 2 3 L P ) I(P)=\frac{M}{L}\int_{-P}^{L-P}x^2dx=\frac{M}{3}(L^2+3P^2-3LP)

The angular acceleration as a function of P is

α ( P ) = τ ( P ) I ( P ) = 3 g μ ( L 2 P + 2 P 2 / L ) 2 ( L 2 + 3 P 2 3 L P ) \alpha(P)=\frac{\tau(P)}{I(P)}=\frac{3g\mu(L-2P+2P^2/L)}{2(L^2+3P^2-3LP)}

Differentiating the acceleration with respect to P and setting it equal to zero (to find a minimum or maximum) yields P = L 2 P=\frac{L}{2} 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 \alpha_{max}=1.5 ..... which means ..... T m i n = ω 1.5 T_{min}=\frac{\omega}{1.5}

α m i n = 0.75 \alpha_{min}=0.75 ..... which means ..... T m a x = ω 0.75 T_{max}=\frac{\omega}{0.75}

And thus T m i n T m a x = 0.5 \frac{T_{min}}{T_{max}}=0.5

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