When we move in the same direction, the relative speed of the two vehicles is lower, so we must subtract their speeds, if the result is positive the vehicles approach and if it is negative they move away.
v_relative = v₁ - v₂
When the two vehicles go in opposite directions, the speed is much higher, so we must add their speeds
This shows correctly Newton’s second law, where sum of forces is divided by mass is equal to acceleration. Also mass can’t be negative so F^n is positive.
No you can't cuz,if you put water instead of clock oil in Millikan oil drop your experiment will fail and it won't turn out the way you wanted it to be
