Chapter 6: Energy

6.6 “Waste”

When an object’s energy is converted from kinetic to potential (or vice versa), the object itself does not lose any energy. However, we know from experience that objects can lose energy. For example, when you apply brakes on your bicycle you go from having kinetic energy to having none, once you stop. The energy left your bicycle almost entirely in the form of heat; if you touched your wheel’s rim and the brake pads, they would feel warm.

The heat is generated by friction between the brake pads and the rim. For a more direct demonstration of this, rub your hands together and note that they warm up. We will return to this idea later on, when we talk about work in chapter 10.

The energy lost from a system can be found by finding the change in the system’s total energy:

$\text{Waste} = \text{Final energy} – \text{Initial energy}$

Or, in symbols:

$W = E_f – E_i \tag{6.8}$

Note that waste comes out as a negative number. Even though energy is a scalar quantity, this negative sign has physical meaning; this represents energy leaving the system.

Returning to the money analogy from section 6.1, this is like exchanging your money from one currency to other (for example from US Dollars to Malaysian Ringgits) where the bank doing the exchange takes a small percentage as a service fee. The total amount of currency is the same, it has changed forms, and some of it has left your bank account (and this is represented by a negative value in your transaction history).