I've got a problem from Irodov. There are two stationary fields of force F=ayi and F = axi + byj, where i and j are unit vectors of the x and y axes, and a and b are constants. Find out whether these fields are potential. But how do I know that a field is potential? I've read that I have to calculate the work performed by these forces and if the work depends on the path or not. But why of this?