Answer: 5v/3.
- A v/3
- B v
- C 5v/3
- D 7v/3
Correct answer: C. 5v/3
Explanation: Using elastic collision formulas: v2_after = (2m1 x u1 + (m2-m1) x u2) / (m1+m2). Take toward each other as opposite signs. Let u1=3v, u2=-v, m1=m, m2=2m. v2=(2m x 3v + (2m-m)(-v))/(3m) = (6v - v)/3 = 5v/3.
As a pendulum swings, total mechanical energy (KE+PE) stays constant: at the highest points A and C, all energy is potential (zero speed); at the lowest point B, all of that energy has converted into kinetic energy (maximum speed).
Concept context
Work-energy theorem, potential energy, conservation of energy, collisions.