Consider the following statements regarding Newton's laws of motion, conservation of momentum, and inertia under Mechanics:

1. 1. Newton's first law of motion, also known as the 'Law of Inertia', defines that an object will remain in its state of rest or uniform motion in a straight line unless acted upon by an external force.
2. 2. Rocket propulsion works primarily on the principle of conservation of energy, while the law of conservation of momentum does not apply to it.
3. 3. When a moving bus stops suddenly, the passengers sitting inside lean forward, which is an excellent example of inertia of motion.
4. 4. The force applied on an object is directly proportional to the rate of change of its momentum, and the direction of this force is in the same direction in which the momentum changes (this is Newton's second law of motion).

Which of the statement(s) given above is/are completely correct?