Right!
- Since both the Earth and the bicycle are in the system, the system has gravitational potential energy, and this goes down as the bike goes down.
- There is no change in kinetic energy, because speed is constant.
- There are no external forces, since all the objects that act on each other (the bike and the Earth) are included in the system, so the work done by external forces is zero.
- Since the energy has to go somewhere, this leaves only "dissipated energy" as an option. Indeed, the action of the brakes is increasing the thermal energy content of the bicycle, and this has to be by an amount equal to the decrease in gravitational potential energy.
Now answer this: if we chose the system to be the bicycle only (with you on it), what would the work-energy diagram look like? Choose from the pictures shown below.
Click here to choose diagram (a).
Click here to choose diagram (b).
Click here to choose diagram (c).
Click here to choose diagram (d).
Go back if you want to look at the original question.