Laws of Motion

JEE Main level, single correct, medium

43 of 210

Question 43 of 210

A ball is thrown upward with an initial velocity V0{V}_{0} from the surface of the earth. The motion of the ball is affected by a drag force equal to mγv2m\gamma {v}^{2} (where mm is mass of the ball, vv is its instantaneous velocity and γ\gamma is a constant). Time taken by the ball to rise to its zenith is:

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Answer: (B) 1γgtan⁡−1⁡(γgV0)\dfrac{1}{\sqrt{\gamma g}}{\tan }^{-1}⁡\left(\sqrt{\dfrac{\gamma }{g}}{V}_{0}\right)

Net force on ball,
F=−mg−mγv2F=-mg-m\gamma {v}^{2}
a=−g−γv2a=-g-\gamma {v}^{2}
dvdt=−g−γv2\dfrac{dv}{dt}=-g-\gamma {v}^{2}
∫v=V0v=0dvg+γv2=∫t=0t−dt\int _{v={V}_{0}}^{v=0}\dfrac{dv}{g+\gamma {v}^{2}}=\int _{t=0}^{t}-dt
(1gγtan−1γgv)v=V0v=0=−t{\left(\dfrac{1}{\sqrt{g\gamma }}{tan}^{-1}\sqrt{\dfrac{\gamma }{g}}v\right)}_{v={V}_{0}}^{v=0}=-t
∴t=1γgtan−1(γgV0)∴t=\dfrac{1}{\sqrt{\gamma g}}{tan}^{-1}\left(\sqrt{\dfrac{\gamma }{g}}{V}_{0}\right)

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