A passenger in a moving bus is thrown forward when the bus is suddenly stopped. This is explained :

by Newton's first law 

Newton's laws of motion-

• ​Newton’s first law states that, if a body is at rest or moving at a constant speed in a straight line, it will remain at rest or keep moving in a straight line at constant speed unless it is acted upon by force.
• This postulate is known as the law of inertia. The law of inertia was first formulated by Galileo Galilei for horizontal motion on Earth and was later generalized by René Descartes.
• Before Galileo, it had been thought that all horizontal motion required a direct cause. Still, Galileo deduced from his experiments that a body in motion would remain in motion unless a force (such as friction) caused it to come to rest.
• Newton’s second law is a quantitative description of the changes that a force can produce in a body's motion.
• It states that the time rate of change of a body's momentum is equal in both magnitude and direction to the force imposed on it.
• The momentum of a body is equal to the product of its mass and its velocity. Momentum, like velocity, is a vector quantity, having both magnitude and direction.
• A force applied to a body can change the magnitude of the momentum, direction, or both.
• For a body whose mass m is constant, it can be written in F = ma, where F (force) and an (acceleration) are vector quantities.
• If a body has a net force acting on it, it is accelerated by the equation. Conversely, if a body is not accelerated, there is no net force acting on it.
• Newton’s third law states that when two bodies interact, they apply forces to one another that are equal in magnitude and opposite in direction.
• The third law is also known as the law of action and reaction. This law is important in analyzing static equilibrium problems, where all forces are balanced, but it also applies to bodies in uniform or accelerated motion.
• The forces it describes are real ones, not mere bookkeeping devices. For example, a book resting on a table applies a downward force equal to its weight on the table.
• According to the third law, the table applies an equal and opposite force to the book. This force occurs because the book's weight causes the table to deform slightly so that it pushes back on the book like a coiled spring.