Question

Identify each energy exchange as primarily heat or work and determine whether the sign of AE 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) You may want to reference (Pages 253 - 258) Section 6.3 while completing this problem.

Answer 1

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PART A:

(a)

When first ball (system 1) collides with second ball, this ball transfers it's energy (work is done) (kinetic energy) to the second ball (system 2). This is why, second ball starts motion while first ball comes to rest.

Hence, Energy exchanges as primarily work.

(b)

When a book fall from a height to floor, then it's gravitational potential energy is transferred into kinetic energy (work done). The work is done by the system on the floor.

Hence, Energy exchanges as primarily work.

(c)

When a father pushes his daughter on a swing (the daughter and the swing are the system), the force is applied by father on system. Hence the work is done on the system.

Hence, Energy exchanges as primarily work.

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PART B:

(a)

When first ball (system 1) collides with second ball, this ball transfers it's energy (kinetic energy) to the second ball (system 2). Hence, system loses energy. That means internal energy decreases.

Therefore, Sign of $\Delta E$ is negative for the system.

(b)

When a book fall from a height to floor, then it's gravitational potential energy is transferred into kinetic energy (work done). The work is done by the system on the floor. Hence the system loses it's energy. That means internal energy decreases.

Therefore, Sign of $\Delta E$ is negative for the system.

(c)

When a father pushes his daughter on a swing (the daughter and the swing are the system), the force is applied by father on system. Hence the system absorbs energy in the form of work. That means internal energy increases.

Therefore, Sign of $\Delta E$ is positive for the system.

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