Relativity-I think!! Hi, I'm new here,so I'm kinda nervous. Anyway,I have a big question(big to me atleast!) about inertia. I've been pondering what exactly this thing inertia is-why does a body have inertia just because it has a mass? What capability does "matter" in an object have to try and prevent external forces from acting on it????? I thought that Einstein's E=mcsquared might help-saying that mass has a certain amount of energy may mean that it has energy to oppose external forces--I'm not really sure. On the moon where the gravitational forces are a lot weaker,its easier to lift an object,does it mean that it has less inertia. If it does, inertia is supposed to depend on mass only,so gravitational forces aren't supposed to affect it!!! Please help!!