Final answer:
The phenomenon of feeling pushed into the seat during acceleration or leaning forward during deceleration is explained by Newton's first law, the law of inertia, which states that an object will maintain its state unless acted upon by a net external force.
Step-by-step explanation:
The sensation of being pushed back into your seat when a car or jet accelerates, and the feeling of being thrown forward when it stops, can be explained by Newton's first law of motion. This law, also known as the law of inertia, states that a body at rest will stay at rest, and a body in motion will continue in motion with a constant velocity, unless acted upon by a net external force. When a car suddenly accelerates, your body tends to remain at rest due to inertia, so the seat pushes forward against you, making you feel like you're being pushed back. Similarly, when the car slows down, your body wants to keep moving due to inertia, causing you to lean forward as the car's velocity decreases.
Despite these sensations, there is no real backward or forward force applied to your body; it's the inertia of your body resisting the acceleration or deceleration of the car that results in these feelings. In contrast, Newton's third law of motion, which describes action-reaction pairs, is different. It explains how forces come in pairs; for instance, as a car accelerates, its wheels push back on the ground, and the ground pushes the car forward.