> These equations tell you that like masses attract and unlike masses repel. We don’t normally talk about this because for all we know there are no negative gravitational masses, but you can see what happens in the Newtonian limit.<p>I feel like I am misunderstanding what the author wants to say here. It seems to me that this would only be the case if you change the sign of the gravitational mass (F[-m_1,-m_2]=F[m_1,m_2]), but not of the inertial mass?<p>In the Newtonian case you get that the force is proportional to m_1<i>m_2, so +</i>+=+, +<i>-=- and -</i>-=+, but then F=ma flips the direction of the acceleration, right?<p>+<i>+ gives F>0 and a>0, so attraction.<p>-</i>- gives F>0, but negative inertial mass yield a<0 and hence repulsion.<p>+*- gives F<0, so the positive inertial mass sees a<0 and is repelled, while the negative inertial mass sees a>0 and is attracted.<p>What am I missing here?