Because his body partakes of the motion of the wagon and moves on after his feet are stopped by the ground, so that he falls forwards.
How can he prevent falling?
He can prevent falling by jumping in the same direction as that in which the wagon is moving.
When a stone is dropped from the top of a mast of a vessel, where does it fall?
The stone falls at the bottom of the mast.
Why does the stone fall at the bottom of the mast?
Because it partakes of the motion of the vessel, and is carried forward while falling.
When a ball is thrown perpendicularly into the air by a person on a boat, where does the ball fall?
The ball falls back into the person’s hand, although the boat may have carried him several feet while the ball was in the air.
Why does the ball fall back into his hand?