A rectangular tank of mass `m_(0)` and charge Q is placed over a smooth horizontal floor. A horizontal electric field£ exist in the region. Rain drops are falling vertically in the tank at the constant rate of n drops per second. Mass of each drop is m. Find velocityoftank as function of time.

Explanation

Two forces will act on the tank
a. Electrostatic force
b. Thrust force.
let v bet the velocity at any instat. Then,
`F_("net")=QE-mnv`
`or `(m_0+mnt)(dv)/(dt)=QE=mnv`
or `int_0^v (dv)/(QE-mnv)=int_0^t (dt)/(m_0+mnt)`
or `"ln" ((QE)/(QE-mnv))="In" ((m_0+mnt)/m_0)`
or `(QE)/(QE-mnv)=(m_0+mnt)/m_0`
or` v=QE(t/(m_0+mnt))`