I am working on a drone to capture a soccer game. What should be ideal minimum frame rate for the camera? Does it depend on velocity of objects(ball, players) in the game?
There are two distinct types of frame rates and how they're captured and played back.
A progressive frame rate transmits whole images at once, depending on whether you use NTSC or PAL, you'll start on the low end with 25/24 FPS. From there, you can go up as much as you want, but the conventional frame rates are usually 25, 30 and 60. Because you're filming a soccer game, 60fps will give you a more crisp, clear and even slower viewing experience, without actually slowing down the footage.
When it comes to interlacing, it's a bit more complex. TV and broadcasts used and still use interlacing for the most part. With it, you'll only send half the picture, first the even or uneven rows, and after that the other half. Using interlacing, you can safe a lot of data and make the image still look quite good, however, in rapid movements (be it a whip pan or alike), you'll be able to make out interlacing lines which I absolutely detest.
That said, you should most likely opt for 60p (p standing for progressive) or, if you can't get progressive frame rates, 60i or 120i. Interlaced frame rates get confusing, because for example 25i and 50i is practically the same, only the name differs, since you're sending 50 half-frames a second, but 25 whole frames.
A few considerations - the key one is - where is the final video going to be shown? If it's on television, different regions require different frame rates. If it's online, e.g. youtube can show content up to 60 fps.
Are you planning to slow down any of the footage? For example, if your delivery frame rate is 25fps (as it would be for UK TV), you could shoot at 50fps, and slow down by 50% without losing smoothness of motion when viewed at 25fps. Shoot at 100fps, and you could slow down to 25% original speed etc etc.
Finally, in many countries there are strict rules about flying drones over professional sports games. There are also safety issues about flying drones over anyone. There are ways to mitigate these issues and work around them, but definitely also worth considering when shooting video from drones.