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JEE Advanced 2011 · Laws of Motion

A ball of mass (m)(m) 0.5 kg0.5\ \text{kg} is attached to the end of a string having length (L)(L) 0.5 m0.5\ \text{m}. The ball is rotated on a horizontal circular path about vertical axis. The maximum tension that the string can bear is 324 N324\ \text{N}. The maximum possible value of angular velocity of ball (in radian/s\text{radian/s}) is

Answer: D. 3636

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Written solution

Step 1: Identify the given parameters.

The mass of the ball, the length of the string, and the maximum tension are given.

m=0.5 kgm = 0.5\text{ kg} L=0.5 mL = 0.5\text{ m} Tmax⁡=324 NT_{\max} = 324\text{ N}

Step 2: Set up the equations of motion for the conical pendulum.

Let θ\theta be the angle that the string makes with the vertical axis, and let rr be the radius of the horizontal circular path.

r=Lsin⁡θr = L\sin\theta

The vertical component of the tension balances the weight of the ball.

Tcos⁡θ=mgT\cos\theta = mg

The horizontal component of the tension provides the necessary centripetal force.

Tsin⁡θ=mω2rT\sin\theta = m\omega^2 r

Step 3: Simplify the tension equation.

Substitute r=Lsin⁡θr = L\sin\theta into the centripetal force equation.

Tsin⁡θ=mω2(Lsin⁡θ)T\sin\theta = m\omega^2 (L\sin\theta)

Divide both sides by sin⁡θ\sin\theta.

T=mω2LT = m\omega^2 L

Step 4: Calculate the maximum angular velocity.

The maximum angular velocity ωmax⁡\omega_{\max} occurs when the tension reaches its maximum limit Tmax⁡T_{\max}.

Tmax⁡=mωmax⁡2LT_{\max} = m\omega_{\max}^2 L ωmax⁡=Tmax⁡mL\omega_{\max} = \sqrt{\frac{T_{\max}}{mL}}

Substitute the given values into the equation.

ωmax⁡=3240.5×0.5\omega_{\max} = \sqrt{\frac{324}{0.5 \times 0.5}} ωmax⁡=3240.25\omega_{\max} = \sqrt{\frac{324}{0.25}} ωmax⁡=1296\omega_{\max} = \sqrt{1296} ωmax⁡=36 rad/s\omega_{\max} = 36\text{ rad/s}

Answer: 36 rad/s36\text{ rad/s}

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