Physics started with mechanics – the science of machines, forces and motion. Newton described gravity as glue that held everything together; a force of attraction that was felt mutually by all particles everywhere in the universe. The force of attraction is very strong between the earth and massive objects close to it and it is very weak between earth and tiny objects which are far away. The force of attraction between two objects depended on the distance between their centers, their two masses and some constant which is called Newton's gravitational constant.
The first great flowering of physics came in when Newton refined Galileo's seminal work with metal balls, by studying how objects moved in response to any force, not just gravity. Newton summarized their behavior in three simple laws.
The first law says that in a world where there are no forces to push things around, an object that is not moving will remain at rest forever, where as an object that is moving will keep moving forever along a straight line and at a constant speed. According to second law, an object pushed by a force, either accelerate or decelerate depending on how the force is applied.
Newton's second law of motion was an example of cause-and-effect relationship that we seek to recognize in the phenomena we observe.
The third law says that if two objects collide with each other, each will feel the force of collision equally but in opposite directions.