Identify each energy exchange as primarily heat or work and determine whether the sign of ΔE 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 falls 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) When the two balls collide, the first billiard ball transfers its kinetic energy to the second ball which consequently starts rolling as the system ball comes to rest. Therefore, the decrease in internal energy takes places for both the system and energy transfer. Conclusively, Work done and ΔE are negative

(b) The drop of a book in this case system from a height transfers the potential energy to kinetic energy. Therefore, both the work is done by system on the floor and internal energy of the system decreases. In conclusion, Work done and ΔE are negative

(c) A father pushing daughter on swing which is system implies that he applied some force therefore transferring energy from the source to system. This means the internal energy of the system increases and so both the work done and ΔE are positive