WAEC 2023 Physics · Question 19
A ball of mass 0.8 kg is dropped from a height, H. Just before hitting the ground, its velocity is 5.0 m/s. Determine H. [g = 10 ]
Options
- A.1.25m
- B.4.00m
- C.6.25m
- D.1.60m
Answer: A. 1.25m
Explanation written with AI and checked against the answer key
Analyze the question: A ball is dropped (so it starts from rest, u = 0). It hits the ground at v = 5.0 m/s. We need the height H it fell through. Concept: For free fall, we can use either the equation of motion or conservation of energy. The mass (0.8 kg) is extra information and does not affect the answer, because it cancels out. Solution: Using the equation of motion: v² = u² + 2gH 5² = 0 + 2(10)H 25 = 20H H = 25 / 20 = 1.25 m Check using energy conservation: Potential energy lost = Kinetic energy gained mgH = ½mv² gH = ½v² 10H = ½(25) = 12.5 H = 1.25 m ✔ Both methods give H = 1. Answer: Option A, 1.25 m Tip: In free-fall questions, the mass cancels out, so ignore it when it is given as a distractor. Remember the shortcut H = v²/(2g). Here that is 25/20 = 1.25 m.AI can make mistakes. Report anything that looks wrong.
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