Before Einstein, physicists believed that the behaviour of almost everything was controlled by three rules called “Newton’s Laws of Motion”. The basic point was to relate the motion of objects to the “forces” that act on that object. These forces picked out certain reference frames as special (Inertial frames).
Newton’s First Law of Motion
This law is concerned with what happens when there are no forces. It states, that, there exists a class of reference frames (called inertial frames) in which an object moves in a straight line at constant speed (at time t) if and only if zero (net) force acts on that object at time t.
Therefore, we can define any frame in which the above description is true as inertial. Consider this case: If object A’s frame is inertial, then object A will measure object B to have constant velocity ( at time t) if and only if zero force acts on object B at time t.
Newton’s Second Law of Motion
The Newton’s second law deals with what happens to an object that does experience a new force. Newton’s second law of motion can be stated as, in any inertial frame, (net force on an object) = (mass of object) X (acceleration of object)
F = ma
Acceleration (a) = dv/dt, the rate of change of velocity with respect to time.
The ‘in any inertial frame’, phrase, implies that the acceleration must be measured relative to an inertial frame of reference.
Newton’s Third Law of Motion
The Newton’s third law deals with the relationship between two forces, and can be stated as:
Given two objects (A and B), we have (force from A on B at some time t) = – (force from B on A at some time t)
Also Read: The Limitations of Classical Physics
