Imagine, There's a ball in deep space traveling with a constant velocity of 25m/s and is having mass 25kg. Now as I said, ball is traveling with a constant velocity that simply means acceleration=0 and now if we calculate the force with F=ma we will end up with 0. But if that ball collide with a Space Shuttle it will produce a change in velocity of shuttle, which can be only be done by force. So, how the hell can we say that Force is equal to zero in this case. If you are weak in imagination try considering this as an example:- One day, I am driving my 1814.369kg or simply 4000 pounds car on a straight road which is applying no friction to my car's with a constant velocity of 35m/s and suddenly Mr.Einstein comes in front of my car(even I don't know how) I thought of Newton's formula F=ma according to which Mr.Einstein will not die but will start traveling with me(As their is no friction being applied by the road) but when I actually hit the Mr.Einstein, he died. If you don't believe ask your friend to hit you with a car with a constant velocity of 30km/hr and If you don't die=> Congrats!!! Next Time I am going to use speech to text for writing long questions.