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# Q:A body is thrown up in a lift with a velocity ‘u’ relative to the lift and the time of flight is t. Show that the lift is moving up with an acceleration (2u-tg)/t.Using the concept the relative motion and eqn. v=u+at, I’ve managed till a= (u+v-tg)/t but i am not able to conclude how final velocity (i.e., v) before striking the ground equals the initial velocity (u) with which the ball was thrown, since the lift is accelerating upwards (which means that the ball will hit the ground of the lift before aquiring its initial velocity).

Arun Kumar IIT Delhi
7 years ago
$v_{b,rel}=u\vec j\\ a_{gravity}=-g\vec j\\ a_{b,rel}=-g\vec j-a\vec j\\ T_{flight,rel}=2*u/(g+a)=t\\ =>a=(2*u-gt)/t$

Arun Kumar
IIT Delhi