In 1666, plague struck England, and the university was shut down. Newton went home to the family farm, and spent his time worrying about the problem of how objects move when a force is applied to them, particularly the force of gravity. Newton's laws of motion and law of gravity, for instance, are used to compute the trajectories of interplanetary spacecraft like the Cassini spacecraft, now on its way to Jupiter, then Saturn. No matter what units you express it in, speed is equal to the distance traveled divided by the time taken to go that distance. An object traveling in a circle at a constant speed is being accelerated, since its direction of travel is constantly changing.
Gravity is an example of a force that is always a pull.
Newton's First Law tells us what happens when there is no net force acting on an object. Newton's Second Law, given below, tells us what happens then. The acceleration of an object is directly proportional to the force acting on it, inversely proportional to the mass of the object, and in the same direction as the force. In the above equations, a is the acceleration of an object, m is the mass of the object, and F is the net force acting on the object. If you know the mass and acceleration of an object, you can compute the force which is being applied to it.