AIIMS UG 2013 Mock Test

If we study the vibration of a pipe open at both ends, then which of the following statements is not true?

A source of unknown frequency $f_u$ gives $4\text{beats/s}$ with a known source of $250\text{Hz}$. The second harmonic of $f_u$ ($2f_u$) gives $5\text{beats/s}$ with a source of $513\text{Hz}$. The unknown frequency is:

Two pith balls carrying equal charges are suspended from a common point by strings of equal length. The equilibrium separation between them is $r$. Now the strings are rigidly clamped at half the height ($y/2$). The equilibrium separation between the balls now becomes:

$A,B$ and $C$ are three points in a uniform electric field $\vec{E}$ directed along the positive x-axis. $B$ is further left than $A$, and $A$ is further left than $C$. The electric potential is:

A wire of resistance $4\Omega$ is stretched to twice its original length. The resistance of the stretched wire would be: