Hey, first time here. I'm currently reading up on Physics, preparing for a selection test by myself. I need some help in understanding Newton's gravitational mass and inertial mass. Are they different? I read that there are some difference, and these masses ARE slightly different. Any reason why? I also met an interesting question. "Our universe is expanding, which means that the large masses in it are moving away form each other. One theory states that this expansion is due to the gravitational mass of an object being very slightly different from its inertial mass. Which mass would have to be larger in order to produce an expanding universe? Justify ur answer" Can someone please help?