Newton Law Motion first second third Inertia Fma Mg Forces Action Reaction

Newton’s Law of Motion were first compiled and published on 5th July 1687.  As most of us know, Newton has 3 basic laws of motion.

Newton’s First Law also known as the Law of Inertia states that “If an object is at rest, it tends to stay at rest or if the object is moving, it will continue to move in a straight line unless some external force changes the object’s motion.” The property of inertia is related very close to Newton’s First Law. Inertia is the tendency of objects to resist any change in its motion or state of rest. To understand this, let us take the example of passenger in a bus. Passengers in a bus will lurch backwards when a stationary bus starts to move forward. Why? When the bus starts to move forward, the inertia (the tendency of the passengers to remain at rest) in the passengers made them to remain at their initial positions. Now, when the bus halts suddenly, the passengers move forward. Why? It is the same reason that is, the inertia (the tendency to resist any change in its motion) in the passengers made them to remain at their state of motion.

Newton’s Second Law states that “The rate of change of momentum of a moving object is proportional to and in the same direction as the net force acting on it.” Some of us may be wrong when we say that Newton’s Second Law is F=ma. F=ma only supports the law and is used for calculations related to Newton’s Second Law.

Newton’s Third Law is very simple. It states that “To every action, there is an equal but opposite reaction.” For example, when a person tries to jump from a boat to the jetty, his legs apply a force which pulses the boat backwards (the action). This action in turn produces a reaction force pushing him forward. It does not matter which is the action force or the reaction force as both exist together only in a direction. Meaning to say, that in a state of equilibrium, the vertical and the horizontal resultant force (could be up and down, or left or right depending on the position of the object) are zero.

Take an example of a book placed horizontally on a table. The normal force, usually labeled as N, upwards is the force of the horizontal surface on the object. The weight of the object, W=mg is downwards. Notice that the two different forces are in opposite direction. The equation for this can be written as:

N – mg = 0

(negative sign for opposite direction)

Therefore, N = mg