alleryA constant force of 6 N acts for 4 s on a 2 kg body initially at rest. The kinetic energy of the body at the end of 4 s is:
A36 J
B72 J
C108 J
D144 J
Answer & Solution
Correct answer: D. 144 J
1. Acceleration a = F / m = 6 / 2 = 3 m s⁻².
2. Velocity after 4 s: v = u + a t = 0 + 3 × 4 = 12 m s⁻¹.
3. KE = ½ m v² = 0.5 × 2 × 144 = 144 J.
4. Alternative via impulse: J = F·t = 24 N·s = m v ⇒ v = 12 m s⁻¹ ⇒ same KE.
5. Distractor 36 J = ½·6·12 (confusing impulse and work).
_Source: syllabus Physics Ch 4 "Laws of Motion", §4.5 + Impulse, p. 55_
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