A ball is thrown vertically upward with a speed of 40 m/s. The time taken by the ball to reach the maximum height would be approximately
- A.2 s
- B.3 s
- C.4 s
- D.5 s
Correct answer
Explanation
The correct answer is C, 4 s. The ball keeps slowing down by about 9.8 m/s every second until its speed becomes zero at the top, which takes 40 ÷ 9.8 ≈ 4 seconds.
Use the first equation of motion, v = u − gt. At the highest point the final velocity v is zero, the initial velocity u is 40 m/s and g is 9.8 m/s². So 0 = 40 − 9.8t, giving t = 40/9.8 ≈ 4.08 s, or about 4 s. With g taken as 10 m/s² the answer is exactly 4 s. The maximum height reached is u²/2g, about 80 m, and the ball takes the same 4 s to fall back, so its total time of flight is about 8 s.
A is wrong because after 2 s the ball is still rising at about 20 m/s.
B is wrong because after 3 s the ball is still moving up at about 10 m/s.
D is wrong because by 5 s the ball has already passed the top and is falling.
Exam tip: time to reach the top is u/g; total time of flight is 2u/g.
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