We know from our daily activities that an object must be pushed in order to move it. If the push is given to some extent, the movement will be only to that extent. The object stops moving further if there is no force. But, it is not true that force is essential to keep it in constant motion. Generally, frictional force acts as hindrance for a moving object and tends to stop it. Any earth bound object can be stopped from moving using frictional force. According to the Newton’s first law of motion, an object continues to be in constant motion until it is triggered by an external force. The rotation of earth on its axis also does not have any counteracting forces to stop it from moving. The tidal force of the moon might have little effect on the rotation of the earth. But, that force is too small and has negligible effect on earth’s rotation. But, what caused the earth to start its rotation?
There is a theory behind rotation of the earth. It is said that a supernova has sent shockwaves through a cloud of molecular hydrogen. The small molecular blocks with their own momentum combine with one another gradually to form larger blocks. The momentum of the larger chunks will have all the small momentums added up. According to the law of conservation of momentum, the solar nebula has attained its spinning form as it is seen now. The planets were arranged gradually due to gravitational forces. The planets increased their size due to addition of some more moving particles and their momentum added further.
There is another explanation regarding the rotation of earth given by a scientist at Jet propulsion laboratory. The universe was filled with dust and gas initially. The dust cloud started getting flattened into a big disk that was rotating very fast. The big disk due to gravitational force became smaller eventually. As the disk size reduces the speed of rotation increases. The Solar system started forming from the disk with the Sun at its center. The rest of the disk divided into several clumps to form planets, satellites, asteroids and comets each of them with their own movement retained.