Newton’s First Law of Motion
“In the absence of unbalanced force (net forces), a body at rest will remain at rest and a body in uniform motion will remain in uniform motion in a straight line.”
Experiments shows that when no not force acts on a body the body either remain at rest or moves with uniform velocity in a straight line. It means that once a body has been set in motion no net force is needed to keep it moving which is incorrect. A foot ball moving on ground comes to rest after covering some distance, which seems to be invalid according to the statement of Law.
Actually there are external forces acting on it i.e. air resistance, friction and gravity, if we eliminate completely these forces the football would never slowdown and would need no force to sustain motion. The tendency of a body to remain at rest or uniform motion is called inertia and Newton first Law is often called Law of inertia because a body initially at rest will remain at rest become of inertia or in uniform motion will remain at uniform motion because of inertia. The mass of a body determines. The resistance to a change of its state of rest or uniform motion.
(i) A physics book at rest on horizontal table top has two forces acting on it. The downward force i.e. weight and an upward supported force exerted by the table top, so the net force is zero its remains at res.
(ii) If the net force acting on a body is zero a body in uniform motion will continue its uniform motion.
(iii) A passenger feels jerks in the backward direction when the bus suddenly starts because of inertia.
(iv) A passenger feels jerks in the forward direction when the bus suddenly stops.