Newton’s first law of motion1. Gives a definition of (zero) force2. Defines an inertial frame.Inertial frame: If the relative velocity between the two referenceframes is constant, then the relative acceleration between the tworeference frames is zero, and the reference frames areconsidered to be inertial reference frames. The inertial frame is thensimply a frame of reference in which the first law holds.The first law does not hold in an arbitrary frame. For example, it fails in the frame of arotating turntable.Zero Force: When a body moves with constant velocity in a straightline, either there are no forces present or the net force acting on thebody is zero . If the body changes its velocity, then there mustbe an acceleration, and hence a total non-zero force must be present.Velocity can change due to change in its magnitude or due to changein its direction or change in both.i0iF SS’Rrr ‘‘ , dRr r vt vdt Galilean transformationIs Earth an inertial frame?Newton’s Second law of motion:If any force generates a change in motion, a double force willgenerate double change in the motion, a triple force will correspondto triple change in the motion, whether that force is impressedaltogether and at once or gradually or successively.Suppose that a force is applied to a body for a time interval t . The impressedforce or impulse produces a change in the momentum of the body,The instantaneous action of the total force acting on a body at a time t isdefined by taking the mathematical limit as the time interval t becomessmaller and smaller,Fm1m a1 1 Fm2m a2 21 22 1m am aInertial massChange of motion is described by the change in momentum of body. For apoint mass particle, the momentum is defined asp mv total d dv F mv mdt dt Inertial mass Gravitational mass

the law of action-reaction