MOTION AND REST
If the position of a body changes with time ,the body is in the state of motion. If the position of the body does not changes with time it is said to be at rest.
ABSOLUTE MOTION
when the motion of a body is observed with respect to a point which is stationary in space is called absolute motion. But in universe no point is stationary. So absolute motion is an imaginary concept.
MOTION IS RELATIVE ie we can define motion relative to other bodies.
FRAME OF REFERENCE
A tree on the ground is at rest relative to earth. But it is in motion relative to sun.So in order to describe motion we first select one frame
If we imagine a co ordinate system attached to a rigid body and we describe the position of any particle in space relative to it. Then such a co-ordinate system is called frame of reference. for location of the object the position vector are drawn from the origin of co-ordinate system. The origin may coincide with the position of observer or not But an observer uses a frame of reference which is stationary relative to him.
CARTESIAN COORDINATE SYSTEM
For the location of an event in a frame of reference we require its position and time of occurrence. Therefore 4 coordinates are required(x,y,z,t). It is called space-time frame of reference.We select a frame of reference in which the motion is easy to describe.
Example
1)A man sitting in the train throw a ball vertically upwards, with respect to an observer sitting in train the path of ball is vertically straight line. But an observer outside the train the path of ball is parabolic.
The frame of observer in train is simple with describing the motion of the ball
2)To describe the motion of planets in the frame with respect to earth is very complicated. But the frame situated in on sun is very simple. Now the motion of all planets are circular
3)Motion of a point on a slow moving wheel of a car. If the frame is on the center of wheel, then the motion of point is circular. But frame situated on earth the motion of point is cycloid.
LIMITATIONS OF NEWTONS LAW
According to Newton's first law a body continues its state of rest or uniform motion in a straight line unless compelled by an external force.
This law violates when the observer is in an accelerated frame. Suppose if the object is at rest and the observer is on an accelerated frame, he can observe the object with negative acceleration and he at rest. A fictitious force is acting on the body.
So Newton introduced an inertial frame , which is not accelerated relative to fixed stars and his laws are valid only in inertial frame.
