State Newton’s first law of motion. Explain it with the help of suitable examples. 

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.