Final answer:
When you lean backward as the bus accelerates, it's due to inertia, as explained by Newton's first law, not an actual force pushing you backward.
Step-by-step explanation:
When you are standing in a bus that suddenly accelerates, you lean backward because of inertia, which is described by Newton's first law of motion. This law states that an object at rest tends to stay at rest, and an object in motion tends to stay in motion with the same speed and in the same direction unless acted upon by an unbalanced force. In this scenario, your body wants to remain at rest while the bus moves forward. Therefore, the sensation of being thrown or leaning backward is not due to a real force pushing you backward, but rather the forward acceleration of the bus and your body's tendency to stay in its original state of motion.
This concept also applies to experiences like being pushed back into the seat in an accelerating jet or feeling a force during a sharp turn in a car. In these situations, the centrifugal force is a apparent force that you feel, while in reality, no such force is pushing or pulling on you.