A particle of mass `m` is projected from the ground with an initial speed `u` at an angle `alpha`. Find the magnitude of its angular momentum at the highest point of its trajector about the point of projection.

Explanation

To find the magnitude of the angular momentum of a particle of mass `m` projected from the ground with an initial speed `u` at an angle `alpha` at the highest point of its trajectory about the point of projection, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Components of Velocity:**
At the highest point of the trajectory, the vertical component of the velocity becomes zero, while the horizontal component remains unchanged. The horizontal component of the initial velocity can be calculated as:
\[
u_x = u \cos(\alpha)
...