Question

A block of mass m having a charge q is placed on a smooth horizontal table

Answer 1

your question is incomplete. A complete question is ---> A block of mass m having charge q is placed on smooth horizontal table and is connected to a wall through an unstressed spring of spring constant k . A horizontal electric field is switched on. Find the amplitude of the resulting SHM on the block.

solution : mass of block = m

charge on block = q

spring constant of connected spring to block and wall = k

horizontal electric field = E

if horizontal electric field applied, an accelerating force acts right direction  on the block. This force is balanced by spring force because spring is also connected to block. Let x is maximum extension of spring.

so, spring force = electrostatic force

or, kx = qE

or x = qE/k

we know, maximum extension of spring is not other than amplitude.so, amplitude is qE/k