A ball is thrown straight upward from the ground with an initial speed of 30 m/s. How long does it take for the ball to return to the ground? (Ignore air resistance, take g = 10 m/s^2)
Show the answer and explanation
Correct answer
B. 6 s
Principle or equation
For vertical motion under constant acceleration, the time to reach maximum height is v0/g, and the total flight time is twice that because the upward and downward paths are symmetric.
Why this answer is correct
The ball's velocity becomes zero at the highest point. Using v = v0 - g t, set v = 0: 0 = 30 - 10 t, so t = 3 s to go up. The total time to return to the ground is 2 * 3 = 6 s.
Example
If a ball is thrown upward at 20 m/s, time up = 20/10 = 2 s, total flight = 4 s.
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