Figure , Scheme of volumetric force in liquid Newton defined force as the gradient of speed momentum over time. As discussed above, it can describe the collision behavior of objects like Eq. 14. However, it cannot explain the forces stemming from the temperature gradient and VG gradient. Universal gravitation exists, but it is an integration of various forces. Sun created a temperature gradient in the solar system. Hence, once an object enters the solar system, the Sun donates an attractive force to the object because the Sun sees a decreasing temperature gradient (Eq. 16). However, the object records a repulsive force. Hence, it rotates in the quest of two forces. Linear motion is so tricky in the universe.