A stone of mass 300g is released from rest from the top of a bulding of height 100m. Determine the kinetic…

Analyze the question: A stone falls from rest. We need the kinetic energy gained after it has fallen a quarter of the building's height.

Concept: By conservation of energy (ignoring air resistance), the kinetic energy gained equals the potential energy lost:

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

Solution:

• Mass: m = 300 g = 0.3 kg
• Distance fallen: h = ¼ × 100 m = 25 m
• g = 10 m/s²

KE = mgh = 0.3 × 10 × 25 = 75 J

Check using kinematics: v² = u² + 2gh = 0 + 2(10)(25) = 500 KE = ½mv² = ½ × 0.3 × 500 = 75 J ✓

Answer: C. 75.0J

Tip: Watch the units. Always convert grams to kilograms before calculating (300 g = 0.3 kg). Using 300 directly would give 75,000 J, which isn't an option. Also note that "a quarter way down" means the distance fallen is 25 m, not the height remaining (75 m). If you used 75 m, you'd get 225 J (option B), which is a common trap.