Rotational MotionMedium

Question

A uniform rod of mass 20 kg and length 5 m leans against a smooth vertical wall making an angle of $60^{\circ}$ with it. The other end rests on a rough horizontal floor. The friction force that the floor exerts on the rod is (Take $g = 10\text{ }m/s^{2}$ )

Options

A.100 N
B.$100\sqrt{3}\text{ }N$
C.200 N
D.$\ 200\sqrt{3}\text{ }N$

Solution

For translational equilibrium

$$\begin{matrix} & N_{1} = Mg \\ & N_{2} = f \end{matrix}$$

For rotational equilibrium

Torque about $A,Mg\frac{L}{2}cos\theta = N_{2}Lsin\theta$

$$\begin{matrix} & \frac{Mg}{2}cot\theta = N_{2} = f \\ & \frac{Mg}{2}cot30^{\circ} = f \\ & \frac{Mg}{2}\sqrt{3} = N_{2} \\ & 100\sqrt{3} = f \end{matrix}$$

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