A body is moving initially at 4m/s. If it's impulse after accelerating at 5m/s$^2$ for 2s is 60Kgms$^{-1}$. What is its mass?

Analyze the question: A body starts at 4 m/s and accelerates at 5 m/s² for 2 s. Its impulse is 60 kg m/s. We need the mass.

Concept: Impulse = change in momentum = F × t = m(v − u)

Since F = ma, impulse = m × a × t.

Solution:

Impulse = m × a × t

60 = m × 5 × 2

60 = 10m

m = 6 kg

Check using velocities:

• v = u + at = 4 + (5 × 2) = 14 m/s
• Change in velocity = 14 − 4 = 10 m/s
• Impulse = m(v − u) = m × 10 = 60, so m = 6 kg ✔

The initial speed of 4 m/s is a distractor. Impulse depends only on the change in velocity, so it does not affect the answer.

(One possible trap: if you took 60 as the final momentum, you would get m = 60/14 ≈ 4.3 kg, which is not an option. The question clearly says the impulse is 60, so the method above is correct.)

**Answer: C. 6 kg.

Exam tip: Impulse = change in momentum = Ft = m(v − u). When a question gives acceleration and time, use m × a × t directly and ignore the initial velocity.