## BdPhO Regional (Dhaka-South) Higher Secondary 2019/5

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**Posts:**38**Joined:**Sat Jan 19, 2019 3:35 pm**Location:**Dhaka, Bangladesh-
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### BdPhO Regional (Dhaka-South) Higher Secondary 2019/5

Imagine a rocket with mass $M$ is moving toward a gas cloud with initial velocity $v$. Naturally the rocket will be colliding with the gas particles and gradually losing its kinetic energy. Now for the sake of simplicity, we can model this gas cloud as tiny particles with very small mass $m$ and density $N$ $particles/m^3$ The cross sectional area of the rocket is $A$. Now find out how much time it will take to decrease the rocket’s velocity to half of its original.

**$The$ $only$ $way$ $to$ $learn$ $mathematics$ $is$ $to$ $do$ $mathematics$. $-$ $PAUL$ $HALMOS$**