Answer
$4.1\times 10^{-3}$ $m/s$
Work Step by Step
The magnetic force acting on the proton balances its weight
Therefore,
$qvB=m_{p}g$
Where, $q$ is the charge of the proton,
$v$ is the velocity of the proton,
$B$ is the magnitude of the magnetic field,
$m_{p}$ is the mass of the proton,
and $g$ is the acceleration due to gravity
or, $v=\frac{m_{p}g}{qB}$
or, $v=\frac{1.67\times 10^{-27}\times 9.81}{1.6\times 10^{-19}\times 2.5\times 10^{-5}}$ $m/s$
or, $v=4.1\times 10^{-3}$ $m/s$
Therefore, the speed of the proton will be $4.1\times 10^{-3}$ $m/s$.
