I've seen inertia described in a few ways and it is simply a mass' resistance to acceleration. But inertia in my mind starts to break down in my mind in relativistic terms because of mass-energy equivalence and the speed of light. As mass and inertia are directly related then shouldn't and object's inertial mass increase along with its relativistic mass at relativistic speeds? Let's say I throw a baseball at 0.9c. Its mass will have increased due to its speed and thus so will its inertia. I'm fine until this point, but there are some questions that pop up. Wouldn't this imply that inertial mass can have a vector component? In the case I gave if the ball was flying at 0.9c along an axis and then I tried to move it orthogonally along that access (along an axis where its velocity was 0) then it would have less inertia than if i tried to accelerate it parallel to it's direction of movement. Next, in the last statement the ball's velocity is relative to some observer. Since relativistic mass is relative to an observer, is inertial mass as well? If so then the amount of force required to change the velocity of an object moving in my inertial frame would depend on the magnitude (and direction as outlined in the last paragraph) of its velocity in that frame. If I were to try to accelerate the ball from a distance, the amount of force required would depend on its velocity. My last question is in regards to relativistic mass and gravity. Again, as velocity increases so too does relativistic mass. What effect does this have on gravity? Would a baseball flying by me at 0.9c pull me towards it with a greater force than one flying by at 10km/h? On the same note, would the ball flying at 0.9c pull me towards it as it has more inertia while the ball flying at 10km/h be pulled towards me as I have more inertia? Does my relative motion to the ball have an effect on its gravitational pull? I guess the biggest question here is does inertia have any other equations or theories besides f=ma? Very little seems to be known about it and the few instances I've seen don't go any further into the question than "it is a property of mass" or, as stated earlier, "it is a mass' resistance to acceleration". Any insight would be helpful in this regard.