A body of mass 2 kg is released from a point 100 m above the ground. Calculate its kinetic energy 80m  from the point of release.

Analyze the question: A 2 kg body is released (from rest) at 100 m above the ground. We want its kinetic energy after it has fallen 80 m from the release point.

Concept: Energy is conserved during free fall. The potential energy lost becomes kinetic energy:

KE gained = mgh, where h is the distance fallen.

Solution:

Take g = 10 m/s²:

KE = mgh = 2 × 10 × 80 = 1600 J

Check using kinematics: v² = u² + 2gh = 0 + 2(10)(80) = 1600, so v = 40 m/s. KE = ½mv² = ½ × 2 × 1600 = 1600 J ✓

Why not the other options?

• 400 J would come from using the remaining height (20 m): 2 × 10 × 20 = 400 J. This is the potential energy at that point, not the kinetic energy.
• 2000 J is the total energy at the start (2 × 10 × 100), which is not among the options anyway.
• The other options don't match any sensible calculation here.

Answer: A. 1600 J

Exam tip: Read carefully whether the distance is "from the point of release" (the distance fallen) or "above the ground" (the height remaining). If it's the distance fallen, KE = mg × (distance fallen). The mass and the fall distance are all you need, and the starting height of 100 m is a distractor.