Question

If collision between A and B is perfectly inelast what is velocity of combined mass just after collision?

• A. 6 m/s
• B. 3 m/s
• C. 9 m/s
• D. 12 m/s

Answer 1

Answer: (B)

3 m/s

Answer 2

Step 1: Find velocity of B at collision

Block B, having a charge –1 C in a uniform electric field E = 10 N/C, experiences a force: F = qE = (–1)(10) = –10 N.

The (magnitude of) acceleration: a = |F|/m = 10/1 = 10 m/s².

Since A is initially at rest (and not moving because no net force acts on it until collision), B covers the distance 1.8 m under constant acceleration. Using s = ½at²

1. 8 = ½·10·t²
   t² = (2×1.8)/10 = 0.36
   t = 0.6 s.

Its speed just before collision is: v = at = 10 × 0.6 = 6 m/s (directed toward A).

Step 2: Find speed of the combined mass immediately after collision

In a perfectly inelastic collision, momentum is conserved. Before collision: Momentum = m₍B₎ v = 1 × 6 = 6 kg·m/s (to the left). Block A is at rest. After collision: Combined mass = 1 + 1 = 2 kg. Let v' be the speed (to the left) after collision. Momentum conservation gives: 2·v' = 6
v' = 3 m/s.

Thus, the speed is 3 m/s.