Giving a Medal.
A history book that weighs 30 N is placed on top of a literature book that weighs 10 N.
What is the force that the history exerts on the lit. book?
What is the force the lit. book exerts on the history book?

Question

Giving a Medal.
A history book that weighs 30 N is placed on top of a literature book that weighs 10 N.
What is the force that the history exerts on the lit. book?
What is the force the lit. book exerts on the history book?

isaiah.feynman · Answer

The history book exerts a force of 30 N. That's the weight of the book, and its directed vertically downwards. The Literature book exerts a force of 30 N as well because the history book is able to sit on it without falling through. (We define weight as the magnitude of the force required to prevent an object from free fall).