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JEE Advanced 2021 · Laws of Motion

One end of a horizontal uniform beam of weight WW and length LL is hinged on a vertical wall at point OO and its other end is supported by a light inextensible rope. The other end of the rope is fixed at point QQ, at a height LL above the hinge at point OO. A block of weight αW\alpha W is attached at the point PP of the beam, as shown in the figure (not to scale). The rope can sustain a maximum tension of (22) W(2\sqrt{2})\,W. Which of the following statement(s) is(are) correct?

Answers: A. The vertical component of reaction force at OO does not depend on α\alpha · B. The horizontal component of reaction force at OO is equal to WW for α=0.5\alpha = 0.5 · D. The rope breaks if α>1.5\alpha > 1.5

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Written solution

Step 1: Determine the angle of the rope.

Since OQ=OP=LOQ = OP = L and ∠QOP=90∘\angle QOP = 90^\circ, the triangle △OPQ\triangle OPQ is a right-angled isosceles triangle.

θ=45∘\theta = 45^\circ

Step 2: Resolve the tension force into components.

The horizontal and vertical components of the tension TT acting at point PP are determined by the angle.

Tx=Tcos⁡45∘=T2T_x = T \cos 45^\circ = \frac{T}{\sqrt{2}} Ty=Tsin⁡45∘=T2T_y = T \sin 45^\circ = \frac{T}{\sqrt{2}}

Step 3: Apply rotational equilibrium about the hinge OO.

The sum of the torques about point OO must equal zero.

Ty⋅L−W⋅L2−αW⋅L=0T_y \cdot L - W \cdot \frac{L}{2} - \alpha W \cdot L = 0 T2L=WL(12+α)\frac{T}{\sqrt{2}} L = W L \left(\frac{1}{2} + \alpha\right) T=W(1+2α)2T = \frac{W(1 + 2\alpha)}{\sqrt{2}}

Step 4: Find the horizontal reaction force ROxR_{Ox} at the hinge.

From horizontal translational equilibrium, the horizontal reaction force equals the horizontal component of tension.

ROx=Tx=T2R_{Ox} = T_x = \frac{T}{\sqrt{2}} ROx=W(12+α)R_{Ox} = W \left(\frac{1}{2} + \alpha\right)

Step 5: Find the vertical reaction force ROyR_{Oy} at the hinge.

From vertical translational equilibrium, the sum of vertical forces must equal zero.

ROy+Ty=W+αWR_{Oy} + T_y = W + \alpha W ROy=W(1+α)−T2R_{Oy} = W(1 + \alpha) - \frac{T}{\sqrt{2}} ROy=W2R_{Oy} = \frac{W}{2}

Step 6: Analyze Statement 1.

The vertical component of the reaction force at OO is independent of α\alpha.

ROy=W2R_{Oy} = \frac{W}{2}

Since ROyR_{Oy} does not depend on α\alpha, this statement is correct.

Step 7: Analyze Statement 2.

The horizontal component of the reaction force at OO is WW when α=0.5\alpha = 0.5.

ROx(0.5)=W(12+0.5)=WR_{Ox}(0.5) = W \left(\frac{1}{2} + 0.5\right) = W

This statement is correct.

Step 8: Analyze Statement 3.

The tension in the rope is 2W2W when α=0.5\alpha = 0.5.

T(0.5)=W(1+2(0.5))2=2WT(0.5) = \frac{W(1 + 2(0.5))}{\sqrt{2}} = \sqrt{2}W

Since 2W≠2W\sqrt{2}W \neq 2W, this statement is incorrect.

Step 9: Analyze Statement 4.

The rope breaks if α>1.5\alpha > 1.5 given the maximum tension Tmax⁡=22WT_{\max} = 2\sqrt{2}W.

W(1+2α)2=22W\frac{W(1 + 2\alpha)}{\sqrt{2}} = 2\sqrt{2}W 1+2α=41 + 2\alpha = 4 α=1.5\alpha = 1.5

Since tension increases with α\alpha, the rope breaks for α>1.5\alpha > 1.5, making this statement correct.

Answer: Statements (1), (2), and (4) are correct.

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