Compute Speed of iPhone when moving on a circular arc?

Consider the following figure:

enter image description here

An iPhon is moving on a circular arc from point A to point B. The radius of the orbit is f. Consider the case that a men stands and holding his arm horizontal to the ground with the iPhone in his hand. Now he starts lowering his arm, while keeping his arm straight.

How do I measure the speed at which the iPhone is moving from point A to point B?

Source: ios

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