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JEE Advanced 2023 · System of Particles and Rotational Motion

A bar of mass M=1.00 kgM = 1.00\,\text{kg} and length L=0.20 mL = 0.20\,\text{m} is lying on a horizontal frictionless surface. One end of the bar is pivoted at a point about which it is free to rotate. A small mass m=0.10 kgm = 0.10\,\text{kg} is moving on the same horizontal surface with 5.00 m/s5.00\,\text{m/s} speed on a path perpendicular to the bar. It hits the bar at a distance L/2L/2 from the pivoted end and returns back on the same path with speed vv. After this elastic collision, the bar rotates with an angular velocity ω\omega. Which of the following statement is correct?

Answer: A. ω=6.98 rad s−1\omega = 6.98\,\text{rad s}^{-1} and v=4.30 m s−1v = 4.30\,\text{m s}^{-1}

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Written solution

Step 1: Calculate the moment of inertia of the bar.

The bar is pivoted at one end OO, so its moment of inertia II about the pivot is given by the formula for a uniform rod pivoted at its end.

I=13ML2I = \frac{1}{3}ML^2

Substitute the given values M=1.00 kgM = 1.00\text{ kg} and L=0.20 mL = 0.20\text{ m}.

I=13(1.00)(0.20)2I = \frac{1}{3}(1.00)(0.20)^2 I≈1.33×10−2 kg m2I \approx 1.33 \times 10^{-2}\text{ kg m}^2

Step 2: Apply conservation of angular momentum.

Since there are no external torques acting on the system about the vertical axis through pivot OO, the angular momentum is conserved.

Li=LfL_i = L_f mur=Iω−mvrm u r = I\omega - m v r

Rearrange the equation to express ω\omega in terms of vv.

Iω=mr(u+v)I\omega = m r (u + v) ω=mrI(u+v)\omega = \frac{m r}{I}(u + v)

Substitute the values m=0.10 kgm = 0.10\text{ kg}, r=0.10 mr = 0.10\text{ m}, u=5.00 m s−1u = 5.00\text{ m s}^{-1}, and I=1.33×10−2 kg m2I = 1.33 \times 10^{-2}\text{ kg m}^2.

ω=(0.10)(0.10)1.33×10−2(5.00+v)\omega = \frac{(0.10)(0.10)}{1.33 \times 10^{-2}}(5.00 + v) ω=0.75(5.00+v)\omega = 0.75(5.00 + v) ω=3.75+0.75v\omega = 3.75 + 0.75v

Step 3: Apply conservation of kinetic energy.

Since the collision is elastic, the total kinetic energy before the collision equals the total kinetic energy after the collision.

12mu2=12mv2+12Iω2\frac{1}{2}mu^2 = \frac{1}{2}mv^2 + \frac{1}{2}I\omega^2

Substitute the known values into the energy equation.

12(0.10)(5.00)2=12(0.10)v2+12(1.33×10−2)ω2\frac{1}{2}(0.10)(5.00)^2 = \frac{1}{2}(0.10)v^2 + \frac{1}{2}(1.33 \times 10^{-2})\omega^2 1.25=0.05v2+6.67×10−3ω21.25 = 0.05v^2 + 6.67 \times 10^{-3}\omega^2

Step 4: Solve for the final velocity vv and angular velocity ω\omega.

Substitute the expression for ω\omega from Step 2 into the kinetic energy equation.

1.25=0.05v2+6.67×10−3(3.75+0.75v)21.25 = 0.05v^2 + 6.67 \times 10^{-3}(3.75 + 0.75v)^2

Expand and simplify the terms to form a quadratic equation.

43v2+30v−925=043v^2 + 30v - 925 = 0

Solve for vv using the quadratic formula.

v=−30+302−4(43)(−925)2(43)v = \frac{-30 + \sqrt{30^2 - 4(43)(-925)}}{2(43)} v=37086v = \frac{370}{86} v≈4.30 m s−1v \approx 4.30\text{ m s}^{-1}

Substitute vv back into the expression for ω\omega.

ω=3.75+0.75(4.30)\omega = 3.75 + 0.75(4.30) ω≈6.98 rad s−1\omega \approx 6.98\text{ rad s}^{-1}

Answer: ω=6.98 rad s−1\omega = 6.98\text{ rad s}^{-1} and v=4.30 m s−1v = 4.30\text{ m s}^{-1}

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