I know for a fact that whenever I am standing on the platform of a bus that starts accelerating from zero I have a tendency to go backwards. The friction on my shoes prevent from going backwards of course and I do the slightest stumble. However, what causes the body in an accelerating bus to move backwards? I know that it happens, but I am having trouble constructintg the logic. Let's forget about friction for the moment, and think of a ball (a sphere) on the floor of the bus. Why does the ball move backwards? Is it the Third Law? Or is it inertia? I tried to get the answer by working backwards: I thought that if the bus makes a sudden stop (sudden negative accleration), the ball will continue to move forward after the bus has stopped. The logic behind this is easy: prior to the deceleration, the ball had been moving at the same velocity as the bus, but while the bus stopped, the ball continued to move at the same velocity at the point of the bus just prior to its deceleration. It's the law of Inertia (a body in motion will have a tendency to stay in motion in an unchanged velocity unless acted upon by external force). So, now that we have the situation of a decelerating bus made clear, what is the situation of the accelerating bus???? I tried working the logic of the decelerating bus backwards but this didn't work out so well. When the ball rolls backwards during the accleration process is it due to inerita (Newton's first law) or Newton's third law??? Can anyone help me on this?