A ball thrown vertically upward returns to its starting point in 4 s. Find its initial speed.

Explanation

To solve the problem of finding the initial speed of a ball thrown vertically upward that returns to its starting point in 4 seconds, we can follow these steps:

### Step 1: Understand the Motion
The ball is thrown vertically upward and returns to the starting point. The total time taken for this motion is given as 4 seconds. This time includes both the upward and downward journey.

### Step 2: Use the Concept of Time of Flight
The time of flight (T) for an object thrown vertically upward and returning to the same point is given by the formula:
\[ T = \frac{2u}{g} \]
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