how to find rotation angle and speed of an object

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i have an application where the the object of interest is rotating, i have to find the speed of the object and rotation angle

here below are few images, full video can be found here

i have an idea to proceed,

• as my object of interest is moving i can just select my area of interest or create a sub image

• after sub image is created i shall calculate centroid of white object

• i shall keep on tracking all points until it completes its revolution

• once done i can find radius

• but how to calculate the rotation angle

i have done basic open cv course of using functions, but i havent come across any functions which calculate angles bewteen three points, i need some help in solving my problem