Newtons Laws of Motion
In particle kinematics, dealt with in the previous chapter, the motion of a particle was described. The description was based on the definitions of some important terms like displacement, velocity and acceleration. However, the basic question that what causes motion ?‘ was not dealt with. Dynamics deals with the answer to the above question in terms of ‘force’ and ‘inertia’. The intimate relationship between force acting on a body and the motion executed by the body is governed by certain were established laws. These laws were propounded by Sir Isaac Newton. They are called Newton1s laws of motion’.
Newton’s First Law - Force and Inertia
The law states that “everybody continues to be in its state of rest or of uniform motion in a straight line unless it is compelled to change that state by an external force impressed on it”.
The first part of the law deals with the state of rest of a body. According to this, everybody preserves its state of rest unless some external force compels it to change its state of rest. The following examples illustrate this part of the law.
1 A duster placed on a table continues to be at rest unless somebody displaces or lifts it by applying a force.
2. A football lying on the ground continues to be in the same position unless somebody kicks ¡t with a force.
The second part of the law deals with the state of motion. According to this, everybody continues in its state of uniform motion in a straight line unless some external force compels it to change that state. This part of the law seems to be violated in our daily life For example:
1. A marble set rolling on the ground comes to rest after travelling a certain distance.
2. A body thrown vertically upwards rises to a certain height and then falls down.
The marble comes to rest because of the force of friction which oppose its motion. If the friction is completely eliminated then the marble will preserve its state of uniform motion.