For the calculation of the speed accurately. First, take two successive images from your video. you can use "free video to jpg converter" to extract every frame. Second, download imageJ software and open the two images. you supposed to know the time interval between them. Then use the scale feature and draw a line for a known distance in the image. this stage determines how accurate your measurement is. it will give you how many pixels per inch/mm depends on your known distance. Finally, find the delta x and delta y of the moving object in pixels (you can easily get the pixel position by hovering the mouse in the image).

For example:

delta t between the two images is 0.001 second

delta x is 114 pixels

delta y is 0 >> object is moving horizontally.

scale: 228 pixels/mm

to find the distance: delta x / scale = 114/228 = 2 mm in 0.001 second

the speed then is distance/delta t = 2/0.001 = 2,000 mm/s or 2 m/s