Identify each of the following energy exchanges as primarily heat or work and determine whether the sign of is positive or negative for the system. a. a rolling billiard ball collides with another billiard ball. the first billiard ball (defined as the system) stops rolling after the collision. b. a book is dropped to the floor (the book is the system). c. a father pushes his daughter on a swing (the daughter and the swing are the system

A) Work (positive)

B) Heat (negative) and work (positive)

C) Work (negative) and heat (negative)

Explanation:

A) This energy change is work given that the first ball is using this energy to move the other one. The system (fist ball) is doing the work so it has + sign.

B) The book loses part of its energy by friction (heat) and part by applying a force to the floor (work). The heat is taken from the system so it has - sign and the work is done by the system and it has + sign.  

C) The daughter is receiving the work done by the father. This work done to the system has - sign. Also, you can say that the system (daughter and swing) loses energy by friction (heat) and because of that it slows down.

Note: the sign of work and heat is defined by convention.