Question

A magnet with a mass of $0.25\text{kg}$ moving with a velocity of $10\text{m/s}$ strikes a metal block of mass $20\text{kg}$ moving with a velocity of $5\text{m/s}$. Both magnet and block are moving in the same direction initially. The magnet sticks to the metal block after the collision. What is the final velocity of the system ?

• A. $8.5\text{m/s}$
• B. $10\text{m/s}$
• C. $7.5\text{m/s}$
• D. $5.06\text{m/s}$

Answer 1

The correct option is D $5.06\text{m/s}$

During collision, only internal forces act on the system, so $F_{\text{ext}}=0$ on system of (block+magnet).
$\therefore P_i=P_f$
Let the system be moving with velocity $v$ in the same direction of motion as initial, after sticking together.
$\Rightarrow m_{\text{mag}}{u}_{\text{mag}}+m_b{u}_b=(m_{\text{mag}}+m_b)v$
$\Rightarrow (0.25\times 10)+(20\times 5)=(0.25+20)v$
$\therefore v=\frac{2.5+100}{20.25}=5.06\text{m/s}$
Hence, the correct option is $\left(d\right)$.