Final answer:
The boy falls backward; the weight of the boy is juxtaposed with and compared to the weight of the purse: "... the boy's weight and the weight of the purse combined" cause him to lose his balance. This physical principle concerning gravity and balance is a concept in physics.
Step-by-step explanation:
The passage you've provided describes a scenario where "the boy's weight and the weight of the purse combined" cause him to lose his balance, suggesting that the central force at play here is gravity and how it affects balance and stability.
This concept is related to physics, specifically to the idea of center of gravity (cg) and balance.
When the combined weight of the boy and the purse shifts, it likely moves their center of gravity in such a way that it is no longer above the base of their support, which in this case would be the boy's feet.
This causes the boy to fall backward, as he cannot maintain balance with the center of gravity shifted too far behind his base of support.