Question

Three blocks are initially placed as shown in the figure. Block $A$ has mass $m$ and initial velocity $V$ to the right. Block $B$ with mass $m$ and block $C$ with mass $4\text{m}$ are both initially at rest. Neglect friction. All collisions are elastic. The final velocity of block $A$ is:

• A. $0.60V$ to the left
• B. $1.4V$ to the left
• C. $V$ to the left
• D. $0.4V$ to the left

Answer 1

The correct option is A $0.60V$ to the left

After $1^{st}$ collision of $A$ and $B$ interchange their velocities (As collision is elastic and masses are equal),


Velocity of $B$
$V_B=\frac{mV+4m(0-V)}{5m}=-\frac{3V}{5}$

After collision with $C$, $B$ has velocity towards $A$.
So, after final collision of $A$ and $B$ velocity gets interchanged

$V_A=0.60V$ to the left