Why does aparticle change its velocity?

Why Does a Particle Change its Velocity?

 Sometimes an object that we are watching-perhaps an automobile, a baseball, or a cat-will change its velocity. It will accelerate. Observation has taught us that when this happens we can always find one or more nearby objects that seems to be associated with that change.We relate the acceleration of a particle then to some interaction between the particle and its surroundings. We are so used  to this that, when we see an object change its velocity without apparent cause,we suspect a trick. If a rolling ball suddenly changes direction, we look for a hidden magnet or an air jet.

            We are given a particle (usually referred to as a body, from now on) whose characteristics (for example, mass,shape,volume,electric charge) we know.

We also know that the locations and properties of all significant nearby objects.That is, we are fully informed about the body’s environment. We want to know how the body will move.

            Isaac Newton (1642-1727), in putting forward his laws of motion and his theory of gravitation, first sol;ved this problem. Here is our plan for following in his footsteps: (!) We introduce the concept of force (a push or pull), in terms of the acceleration given to a selected standard body. (2) We define mass and show how to assign a mass to a body, so that we may understand how different bodies, subject to the same environment, have different accelerations. (3) We find ways to calculate the force acting on a body from the properties of the body and its environment. That is, we look for force laws. (4) We show how multiple forces on a body can be combined to a net force