My book says that when a mass travels in a curved path, like a circle for example, the instantaneous velocity and displacement vectors are both tangent to the path.
I agree that velocity vector is. But no displacement vector. In the previous chapter I learned that vectors like velocity and force is different from displacement in what their vectors mean.
Displacement vector indicates by its head and tail the actual location the object will arrive at.
But velocity does not do this. An object may never arrive at what the arrow head indicates.
For example, in this pic., the vectors are velocity, the one to the very right goes into the ground. But obviously, the object won't ever go there, the place indicated by the arrow head.