Right!

• Since the Earth is excluded from the system, the system by itself (that is to say, you and the bike) has no gravitational potential energy.
• The increase in thermal energy (dissipated energy) for the bike should be the same as before, since that has not changed--all the energy is dissipated at the brake-wheel contact, which is all part of the bike system. (We are neglecting the very slight warming up of the roadway that may result from "rolling friction" or from a small slippage between the road and the tires.)
• With the Earth out of the system, gravity is now an external force, and it does positive work on it, since it points down, and the bicycle's displacement has a downward component.
• The force of static friction, which keeps the bike from sliding downhill, does no work, because its point of application is always instantaneously at rest. This is mentioned in Example 7.7.1 in your textbook (please read that section carefully if you have more questions about this example), and will be discussed at length in Chapter 9.
• The result (7.20), W_{ext, sys} = Δ E_sys, applies here because the system (you and the bicycle) may be assumed to be closed to a good approximation: dissipation happens mostly internally. Thus, the work done by the external force (gravity) equals the change in the system's energy.

Go back if you want to look at the original diagram with the Earth in the system.

Go back if you want to look at the choice of diagrams for this follow-up question.

Good job! You may go on to the next question.