Newton's First Law of Motion Says if an object is in motion, it tends to be in motion

Newton's First Law of Motion always said that if something is in motion, it will be continuing its motion until an unbalanced external force acts on it. Also, if an object is at rest, then it would be continuing its rest until or unless it is acted upon by an unbalanced external force.

Well, that is a theoretical part which we use to read in our books. In order to go for deep study, you can look for various examples such as a ball on rest stays in rest until you move it with the help of your hand. So, when you use your hand, you are providing an external unbalanced force on the ball. That is why, ball starts moving in the direction of force applied.

When a moving bus applies sudden brakes, the passengers which are standing in the bus moves automatically in the direction of moving bus. This happens because bodies of passengers are trying to stay in movement. It is a kind of resistance that bodies are offering to the change in state of movement. This property of resisting the change in the state of movement or rest is called Inertia.