In the current physics, not only the electromagnetic force and gravity are two different forces, but also the electric force and the magnetic force are obviously different. Starting from the discussion of magnetic force, it is believed that the interaction force between matter is not only due to the intrinsic properties of matter, but also related to some influence of the accumulation of surrounding matter. Thus charged and neutral bodies at rest are logically assigned velocities. The concept of momentum is extended, and the product of the mass of an electrically neutral body and its velocity is called mass momentum. The product of the charge of the charged body and its velocity is called charge momentum. It is assumed that there is a force between moving body, that the magnitude of the force is proportional to the product of their momenta, that (when they may be treated as mass points) is inversely proportional to the square of their distance, and that the direction of the force is perpendicular to the direction of motion of the body being acted upon. Based on the above hypothesis, the gravitational equation including the motion velocity is derived, and the magnetic field of the rotating object is also derived. Moreover, based on the above hypothesis, the electromagnetic force is sorted out and explained, and the electric force is given a new explanation, so that the electric force and the magnetic force become the same force. It also is same that the form of the electromagnetic force as the derived equations of gravity. The expression of gravity includes both attractive and repulsive forces, and the repulsive force cannot be observed (what is called gravity has no repulsive force) can be logically explained. Testable predictions are also presented.