Two particles $A$ and $B$ initially at rest move towards each other under a mutual force of attraction. At the instant when the speed of $A$ is $v$ and the speed of $B$ is $2v$,the speed of the centre of mass of the system is

$HIV$,which causes $AIDS$,first starts destroying:

When a horse pulls a wagon,the force that causes the horse to move forward is the force:

Consider the following statements $A$ and $B$ and identify the correct choice in the given answers.
$A$. Tightly bound electrons of target material scatter $X$-ray photon,resulting in the Compton effect.
$B$. Photoelectric effect takes place with free electrons.

$1+\frac{1}{3 \cdot 2^2}+\frac{1}{5 \cdot 2^4}+\frac{1}{7 \cdot 2^6}+\ldots$ is equal to

An electron accelerated through a potential difference $V$ enters a uniform transverse magnetic field and experiences a force $F$. If the accelerating potential is increased to $2V$,the electron in the same magnetic field will experience a force: