A ball is thrown with speed `u` at an angle `theta` with horizontally. After how much time it will move perpendicular to the initial direction of motion.

Explanation

To solve the problem of determining the time after which a ball thrown with speed `u` at an angle `theta` with the horizontal moves perpendicular to its initial direction of motion, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Initial Conditions**:
- The ball is thrown with an initial speed `u` at an angle `theta` with respect to the horizontal.
- The initial velocity can be broken down into its horizontal and vertical components:
- Horizontal component: \( u_x = u \cos(\theta) \)
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