JEE MAIN - Physics (2024 - 1st February Evening Shift - No. 20)
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Explanation
Initial Conditions:
Consider a solid disc of mass $ M $ and radius $ R $ rolling without slipping on a horizontal surface. The center of mass of the disc has an initial speed $ v $.
Kinetic Energy of the Rolling Disc:
For an object that rolls without slipping, its total kinetic energy $ K $ is the sum of its translational kinetic energy and its rotational kinetic energy.
Translational Kinetic Energy:
$ K_{\text{trans}} = \frac{1}{2} M v^2. $
Rotational Kinetic Energy:
The moment of inertia $ I $ of a solid disc about its center is:
$ I = \frac{1}{2} M R^2. $
Since the disc rolls without slipping, the angular velocity $\omega$ is related to the linear speed by:
$ v = \omega R \implies \omega = \frac{v}{R}. $
Therefore, the rotational kinetic energy is:
$ K_{\text{rot}} = \frac{1}{2} I \omega^2 = \frac{1}{2} \left(\frac{1}{2} M R^2\right) \left(\frac{v^2}{R^2}\right) = \frac{1}{4} M v^2. $
Total Initial Kinetic Energy:
Summing these gives:
$ K_{\text{total}} = K_{\text{trans}} + K_{\text{rot}} = \frac{1}{2} M v^2 + \frac{1}{4} M v^2 = \frac{3}{4} M v^2. $
Energy Conservation as the Disc Climbs the Incline:
As the disc moves up the incline, there are no dissipative forces (assuming a smooth incline and no slipping), so energy is conserved. The total kinetic energy at the bottom will be converted entirely into gravitational potential energy at the maximum height $ h $:
$ K_{\text{total}} = M g h. $
Substitute $ K_{\text{total}} = \frac{3}{4} M v^2 $:
$ \frac{3}{4} M v^2 = M g h. $
Canceling $ M $ from both sides:
$ \frac{3}{4} v^2 = g h. $
Thus,
$ h = \frac{3}{4} \frac{v^2}{g}. $
Final Answer:
$ \boxed{\frac{3}{4} \frac{v^2}{g}} $
This corresponds to Option A.
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