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JEE Advanced 2003 · Laws of Motion

What is the maximum value of the force FF such that the block shown in the arrangement, does not move?

Answer: A. 20 N20\,\text{N}

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Written solution

Step 1: Identify the given parameters.

The physical parameters of the block and the surface are given.

m=3 kgm = \sqrt{3}\text{ kg} μ=123\mu = \frac{1}{2\sqrt{3}} θ=60∘\theta = 60^\circ g=10 m/s2g = 10\text{ m/s}^2

Step 2: Resolve the force FF into components.

The force FF is directed downwards and to the right at an angle of 60∘60^\circ to the horizontal.

Fx=Fcos⁡60∘=F2F_x = F \cos 60^\circ = \frac{F}{2} Fy=Fsin⁡60∘=32FF_y = F \sin 60^\circ = \frac{\sqrt{3}}{2}F

Step 3: Set up the vertical force balance.

The normal force NN balances both the weight of the block and the downward component of the applied force.

N=mg+Fsin⁡60∘N = mg + F \sin 60^\circ mg=103 Nmg = 10\sqrt{3}\text{ N} N=103+32FN = 10\sqrt{3} + \frac{\sqrt{3}}{2}F

Step 4: Apply the condition for impending motion.

At the verge of slipping, the horizontal driving force equals the maximum static friction force.

Fcos⁡60∘=μNF \cos 60^\circ = \mu N Fcos⁡60∘=μ(mg+Fsin⁡60∘)F \cos 60^\circ = \mu \left( mg + F \sin 60^\circ \right)

Step 5: Solve for the maximum force FF.

Substitute the known values into the equilibrium equation.

F2=123(103+32F)\frac{F}{2} = \frac{1}{2\sqrt{3}}\left(10\sqrt{3} + \frac{\sqrt{3}}{2}F\right) F2=5+F4\frac{F}{2} = 5 + \frac{F}{4} F4=5\frac{F}{4} = 5 F=20 NF = 20\text{ N}

Answer: The maximum value of the force FF is 20 N20\text{ N}.

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