Question

The bob $A$ of a pendulum of mass $m$ released from horizontal to the vertical hits another bob $B$ of the same mass at rest on a table as shown in figure. If the length of the pendulum is

$1\, m$, what is the speed with which bob $B$ starts moving. (Neglect the size of the bobs and assume the collision to be elastic) (Take $g = 10\, ms^{-2}$)

• A. $4.47 \,ms^{-1}$
• B. $5.47 \,ms^{-1}$
• C. $6.47 \,ms^{-1}$
• D. $3.47 \,ms^{-1}$

Answer 1

Answer: (A)

As the collision is elastic and two bobs have the same mass, therefore bob $A$ transfers its entire momentum to the bob $B$ and bob $A$ does not rise at all. After collision, bob $A$ comes to rest and bob $B$ moves with speed of bob $A$.
$\therefore $ The speed with which bob $B$ starts moving is
$v = \sqrt{2gh} = \sqrt{2 \times 10\,ms^{-2}\times 1 \,m}$
$ = \sqrt{20}\,ms^{-1}$
$= 4.47\,ms^{-1}$