When a running car stops suddenly, the passengers tend to lean forward mainly because of which physical principle?

Introduction / Context:
This everyday observation from road travel illustrates a basic concept in Newtonian mechanics. When a moving vehicle stops abruptly, passengers feel a jolt and their bodies tend to lurch forward. This question checks whether you can connect that common experience with the correct physical law or principle.

Given Data / Assumptions:

• A car is moving forward with some speed.
• Passengers inside the car are also moving forward with the same speed as the car.
• The car is brought to a sudden stop, for example by braking hard.
• Seat belts or other restraints may or may not be present; the important point is the tendency of the passengers.

Concept / Approach:
Newton's first law of motion states that a body continues in its state of rest or uniform motion in a straight line unless acted upon by an external unbalanced force. This property is called inertia. When the car stops suddenly, a large braking force acts on the car, but there is no equally strong force immediately stopping the upper body of a passenger. As a result, the passenger's body tends to continue moving forward due to inertia of motion, making the person lean or lurch forward. Gravity acts downward, not forward, and conservation of mass is unrelated to this effect.

Step-by-Step Solution:
Step 1: Before braking, both car and passengers are moving forward with uniform velocity. Step 2: The driver applies brakes; an external retarding force acts on the car through the wheels and road, quickly reducing the car's speed to zero. Step 3: At the exact moment of braking, the passenger's body does not experience an equally strong backward force at every point, especially in the upper body. Step 4: Due to inertia of motion, the passenger's body tends to continue moving forward even though the car slows down. Step 5: This causes the passenger to lean or lurch forward until restrained by a seat belt, seat, or muscular effort.

Verification / Alternative check:
This phenomenon is symmetric for acceleration: when a car starts suddenly from rest, passengers feel pushed backward because their bodies tend to remain at rest while the car moves forward. Both cases are classic examples used in textbooks to explain inertia and Newton's first law. There is no additional forward gravitational force at work, and the total mass of the system does not change, so the law of conservation of mass is not relevant.

Why Other Options Are Wrong:
Gravitational force pulling them forward: Gravity acts toward the centre of the Earth (downward), not forward along the direction of motion.

Law of conservation of mass: Mass is simply conserved and does not explain changes in motion or the leaning forward effect.
Both B & C: Only inertia of motion is the correct and sufficient explanation here.

Common Pitfalls:
Some learners loosely attribute all motions to gravity or confuse inertia with some mysterious "force." Inertia is not a separate force; it is the natural tendency of bodies to resist changes in their state of motion. Remember that Newton's first law is often called the law of inertia and that vehicle examples (braking, accelerating, turning) are standard illustrations of this principle.

Final Answer:
Passengers lean forward when a running car stops suddenly because of the Inertia of motion of their bodies.