Q. A bullet of mass 10 g moving horizontally with a velocity of 400 m $s^{-1}$ strikes a wooden block of mass 2 kg which is suspended by a light inextensible string of length 5 m. As a result, the centre of gravity of the block is found to rise a vertical distance of 10 cm. The speed of the bullet after it emerges out horizontally from the block will be
A) 100 m $s^{-1}$
B) 80 m $s^{-1}$
C) 120 m $s^{-1}$
D) 160 m $s^{-1}$
Explanation
[Solution] Block speed $= \sqrt{2gh} = \sqrt{2}$ ≈ 1.4 m/s. Momentum: $0.01 \times 400 = 2 \times 1.4 + 0.01v$, so v ≈ 120 m/s. Answer (3).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.