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From the top of a 100 m tall tower, one ball is released from rest. At the same instant, another ball is thrown straight upward from the ground with speed 25 m s^−1. How far below the top of the tower do the two balls meet?

1. 68.4 m
2. 48.4 m
3. 18.4 m
4. 78.4 m

Correct answer: 78.4 m

Solution

The two balls meet when they have traveled the same vertical distance. The ball dropped from the tower falls 78.4 m, leaving 21.6 m above the ground, while the ball thrown upward reaches the same height after traveling 21.6 m upward, confirming that they meet 78.4 m below the top of the tower.

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