Newton's 1st Law Of Motion
INTRODUCTION
In unit II,we studied the kinematics of motion by developing the concepts of displacement velocity and acceleration and by deriving relations between these physical quantities.In this unit ,we shall investigate the cause of motion, and it will be found that body at rest may be set itno motion,when some force is applied on it.The motion of the bodies can be understood by finding out the relation between the force applied on a body and the effect produced by the applied force
INTUTIVE CONCEPT OF FORCE
Force.It is a push or pull which either changes or tends to change the state of rest or of uniform motion of a body.
It is our commom observation that an object such as a chair lying in a room or a vehicle parked outside the house remains at rest unless a push or a pull is given to it .Such an object cannot move at its own.In other words,force has to be applied in order to move an object at rest.Also,if an object is moving along a straight line with some velocity,it is found that force is required to change its direction of motion or the magnitude of its velocity.In other words,force is an agent ,which causes acceleration.However, in certain cases,the accceleration caused by the force may be practically zero.For,example ,if we give push to a stationary truck,it may not move.In such cases ,we say that the force merely tends to cause acceleration.
Hence,force is a push or a pull which produces or tends to produce motion in a body at rest;stops or tends to stop a body which is in motion;increases or decreases the magnitude of veocity of the moving body or changes the direction of motion of the moving body.
INERTIA
Inertia.In the absence of external force,the inability of a body to change its state by itself is called inertia.
As said above,an object such as a chair lying in the room remians at rest , unless an external force is applied on it.It can be set into motion only by applying a force on it.In other words,if we do not apply a force on the stationary chair,it will not move at all.
This inherent property of the objects that they do not change their state unless acted upon by an external force is calles inertia.
For centuries before Galileo,it was the general belief that force is necessary even to keep a body in motion with uniform velocity.With his simple experiments inclined planes,Galileo proved that no force was required to keep a body moving with uniform velocity,provided fricition is not present.Therefore,it is the inherent property of the objects that they remain moving with constant speed along a straight line unless acted upon by an external force.Thus,it is due to inertia that a body cannot change its state of rest or of uniform motion by itself.It is sometimes also called Galileo’s law of inertia .