Suppose you place a ball in the middle of a wagon, and then accelerate the wagon forward. Describe the motion of the ball relative to the ground. - Describe its motion relative to the wagon.

Relative to the ground, the ball will be moving with the wagon, accelerating with it, but more slowly than the wagon at first as it rolls back to the rear of the wagon due to inertia and friction. Once it is at the rear of the wagon and can't roll back further, it will be seen to accelerate at the same acceleration as the wagon, when viewed relative to the ground. Relative to the wagon, the ball will roll back as it will resist change due to inertia when the wagon starts accelerating, due to friction will move in the opposite direction as the wagon, rolling back to the rear.