A classmate argues that the changes in energy that occur in a pendulum are unrelated to the energy changes that a burning log undergoes. She points out that the burning of a log is an irreversible chemical change in matter, while the energy changes in a moving pendulum are constantly reversing, and are examples of physical changes. Evaluate your classmate’s argument.

Explanation:

First of all, let's differentiate between a physical and a chemical change.

A physical change has the following properties

- no new substance is formed

- change is easily reversible

- it mostly involves physical properties (like size)

- examples include burning of candle wax and freezing of water to become ice

A chemical change has the following properties

-new substances are formed

- change is not easily reversible

- it involves both physical and chemical properties (like surface area and reactivity)

- examples include burning of wood and rusting of iron

From the above, it is clear that burning of a log of wood is a chemical change because the wood is burnt in open air (by a chemical process known as combustion) and converted to coal and then ashes. The produced ash cannot be reversed back to wood. The energy change that occurs here (in the combustion) is from chemical energy to heat energy. Which is unrelated to the energy changes that occurs in a swinging/moving pendulum, which is the conversion of potential energy (energy at rest) to kinetic energy (energy in motion). The energy changes in a moving pendulum is not a physical change, which is where the classmate got it wrong.

Hence, she (the classmate) is right about the energy changes been unrelated but she is wrong in her explanation of the process/energy changes involved.