First law of motion:
Its states that a body at rest will remain at rest and a body in motion will continue in motion in a straight line with uniform speed unless it is compelled by some external force to change its state of rest or of uniform motion.
Example:
When a bus starts moving all of a sudden, the head of the passenger sitting in the bus strikes the back of his seat. This can be explained on the basis of the first law of motion. The lower part of the body of the passenger remains in contact with the body of the bus, but the upper part is free. When the bus takes a sudden start, the bus comes in motion and so the lower part of the body, but the head and upper part of the passenger’s body not in contact with the bus, tend to remain at rest and thus, jerked backwards. Similarly, we can explain the jerking of passengers in the forward direction of a bus, when the moving bus stops all of a sudden.