A tennis ball is thrown in the vertically upward direction and the ball attains a maximum height of 20 m. The ball was thrown approximately with an upward velocity of
- A.8 m/s
- B.12 m/s
- C.16 m/s
- D.20 m/s
Correct answer
Explanation
The correct answer is D, 20 m/s. Using v² = u² − 2gh with final velocity zero at the top, u = √(2gh) = √(2 × 10 × 20) = √400 = 20 m/s.
A ball thrown straight up slows down under gravity until its velocity becomes zero at the highest point. The third equation of motion links the initial speed u, the final speed v, the acceleration and the height: v² = u² − 2gh. Putting v = 0 gives h = u²/2g, so u = √(2gh). With g taken as about 10 m/s² and h = 20 m, u comes to 20 m/s (with g = 9.8 m/s² it is about 19.8 m/s, hence "approximately"). The ball would take about 2 seconds to reach the top.
A is wrong because 8 m/s would give a height of only about 3.2 m. B is wrong because 12 m/s would give about 7.2 m. C is wrong because 16 m/s would give about 12.8 m, well short of 20 m.
Exam tip: maximum height h = u²/2g; time to reach the top t = u/g; a ball thrown at 20 m/s rises about 20 m.
Practice Questions
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- B.Martin Luther
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- D.Copernicus
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