A constant force of 5N acts for 5s on a mass of 5kg initially at rest. Calculate the final momentum of the mass.

Analyze the question: A constant force acts on a mass that starts from rest. We need the final momentum after 5 s.

Concept: Impulse equals change in momentum (Newton's second law in momentum form):

F × t = Δp = mv − mu

Solution:

• Force, F = 5 N
• Time, t = 5 s
• Mass, m = 5 kg (it cancels out here, as we only need the impulse)
• Initial velocity, u = 0, so the initial momentum is 0

Final momentum = F × t + initial momentum = 5 × 5 + 0 = 25 kg m s⁻¹

Check using kinematics:

• a = F/m = 5/5 = 1 m/s²
• v = u + at = 0 + (1)(5) = 5 m/s
• p = mv = 5 × 5 = 25 kg m s⁻¹ ✔

Answer: C. 25 kg m s⁻¹

Tip: When a question gives force and time, think "impulse" and use Ft = Δp directly. You can skip finding acceleration and velocity, which saves time in the exam. Also, the mass is extra information here, so don't let it distract you.