Grandmas' rocking chair is empty, grandma isn't there. Grandma's off her rocker and grandma doesn't care. Contraction or possession, placed apostrophe's the key. Grandma's good with grammar knows where should be apostrophe.
So if you're undecided or really are confused. Simply ask my grandma she'll tell you how it's used. Grandmas' rocking chair's still empty, grandma does agree. Contradiction or possession she'll know where should be apostrophe.