Newton's first law of motion states that an object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force. 

(i) First part, says that a body at rest continues in its state of rest. For example, when a bus suddenly starts moving forward, the person falls backward. This happens because, as the bus moves the lower part of his body begins to move along with the bus while the upper part of his body continues to remain at rest due to inertia. Hence, a person falls backward when the bus starts.

(ii) Second part, says that a body in uniform motion continues moving in straight line path with a uniform speed. For example, when a moving bus suddenly stops, a person sitting inside the bus falls forward. As the bus stops, the lower part of his body comes to rest along with the bus while upper part of his body continues to remain in motion due to inertia and so he falls forward.

(iii) Third part, says that a body moving with a uniform speed in a straight line cannot change itself its direction of motion. For example, when a bus takes a sharp turn, a person sitting in the bus gets a force acting away from the centre of the curved path due to his tendency to move in the original direction. He has to hold on to a support to prevent himself from swaying away in the turning bus. 

The three Newton's laws of motion are: 

First Law: An object remains in a state of rest or of uniform motion in a straight line unless compelled to change that state by an applied force.

Second Law: The second law of motion states that the rate of change of momentum of an object is proportional to the applied unbalanced force in the direction of force.

Third Law: The third law of motion states that when one object exerts a force on another object, the second object instantaneously exerts a force back on the first. These two forces are always equal in magnitude but opposite in direction.

Mass and inertia are correlated. Mass of a body is a measure of its inertia. Both of them are directly related. More massive is the body, larger is the inertia or opposition offered by a body to change its state of motion. 

SI unit of mass is kilogram (kg)
SI unit of inertia is kilogram (kg)

The inability of a body to change by itself its direction of motion is called Inertia of direction.

Examples of inertia of direction:

i) A dog chases a hare, the hare runs along a zig-zag path. It becomes more difficult for the dog to catch the hare because dog has more inertia of direction than that of the hare. 

(ii) Consider a stone being rotated at the end of a string in a circle. If the string is suddenly released, the stone flies off tangentially to its circular path.

(iii) As a bicycle moves, the water drops sticking to its tyre start leaving it tangentially.

By observing the motion of objects on inclined planes, Galileo deduced that objects move with constant speed when no force acts on them.

In his first experiment, Galileo studied the motion of marbles on an inclined plane. He observed that when a marble rolls down an inclined plane, its velocity increases, as shown in Fig. 9.6 (a). Here the marble falls under the unbalanced force of gravity. The velocity of the marble decreases when it rolls up the inclined plane (against the force of gravity), as shown in Fig. 9.6 (b). From these observations, Galileo argued that the velocity of a marble rolling on flat horizontal surface should remain constant.

Fig. 9.6. Motion of a marble (a) down the inclined plane and (b) up the inclined plane.

To test his ideas further, Galileo used double inclined plane. He observed that when the inclinations of the planes on both sides are equal, then the marble rolled down from one plane from certain height will climb to the same height on the other plane [Fig. 9.7 (a)].

Fig. 9.7.

Fig. 9.8. Motion of a marble on a double inclined plane.

If the angle of inclination of the right side plane were gradually decreased, then the marble would travel larger and larger distances to reach the same height [Fig. 9.7 (b) and (c)]. Ultimately, if the right side plane were made horizontal, the marble would continue to travel forever to reach the same height from which it was released. No unbalanced force acts on the marble in this case.

The above experiments suggest that an unbalanced force (external force) is required to change the motion of an object while no unbalanced force is needed to keep an object moving with a constant velocity. Galileo concluded the following law of inertia :

A body moving with a certain speed along a straight line path will continue to move with the same speed along the same straight line path in the absence of external forces.

In actual practice, the bodies stop due to the force of friction which always acts opposite to the direction of motion.