Question 21
HardOriginal questionA stone is dropped from rest off a cliff and hits the ground 3 s later. Taking g = 10 m/s² and ignoring air resistance, how high is the cliff?
Show the answer and explanation
CorrectNot quiteAnswer D
Correct answer: D
Explanation
For a fall from rest, height = ½ g t².
= ½ × 10 × 3² = 5 × 9 = 45 m.
Worth noting
30 m/s is the speed on landing (g × t), not the height. 9 is t² alone, and 15 uses t instead of t².
- Concept
- Free fall under gravity
- Typical timeTypical
- 90 s
- Your timeYours
- —
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