Q. An explosion breaks a rock into three parts in a horizontal plane. Two of them go off at right angles to each other. The first part of mass 1 kg moves with a speed of 12 m s⁻¹ and the second part of mass 2 kg moves with 8 m s⁻¹ speed. If the third part flies off with 4 m s⁻¹ speed, then its mass is
A) 3 kg
B) 5 kg
C) 7 kg
D) 17 kg
Explanation
[Solution from source] Initial momentum is zero. The first two momenta are 1 × 12 = 12 and 2 × 8 = 16 kg m s⁻¹ at right angles, giving a resultant √(12² + 16²) = 20 kg m s⁻¹. The third part must carry 20 kg m s⁻¹ in the opposite direction, so m = 20/4 = 5 kg. Answer (2).
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.