I have an object that is moved around and, on event occurrence, is animated.

Right now I am using an image for the object and a sprite sheet for the animation.

Is it better to have also the actual object image inside the sprite sheet?
Does this difference matter for a) performance and b) e.g. hiding the actual object while animation is playing?
How does this matter when an object is a physics object, are there any differences in the physics world if I deal with instances from sprite sheets versus single images?

Also, is it better to have one biiig spritesheet with all the animation stuff in it or several smaller ones with just the frames for one animation or is there no difference?

Just wonder what approach is best, thanks for hints and insight.