Energy is an abstract concept. We cannot see it, but we can utilize the concept to explain complex phenomena.
For example, you work out, control your food intake, and lose weight over time.
Maybe this example is common sense to everyone that doesn’t really require the energy concept to explain it. The gist is you used more energy than you take in, and you burn fats to generate the energy to work out.
If you decide to dig deeper, here is just a glimpse. The chemical energy stored in your body fat is converted to kinetic and thermal energy when you work out. When you take in food, the chemical energy in food is converted into chemical energy in your body. Some are spent for your body’s daily operation, some are spent to work out, the remaining will be stored as fat. When you used more than you take in, your weight goes down. (This is a pure energy consideration, no genetics involved.)
——————————-
What one should do in discussing energy conversion?
First, one needs to decide the system of interest. It is like setting boundaries. Without boundaries, there is no consistent description of energy conversion. The second thing to decide is the time interval.
Here is an example: Ball is released from rest from a height h to just before it touches the ground. Neglect air resistance.
Time interval: From release to just before it touches the ground
System of interest: Ball only
Energy conversion: Gravitational force of the ball does work to increase the kinetic energy of the ball.
——-
This is not the only system we can consider.
Another system of interest (with the same time interval): Ball and Earth only
Energy conversion: The gravitational potential energy of the ball is converted to its kinetic energy during the fall.
——-
Clearly, there is a difference in describing the energy conversion for the same situation. However, it does not affect the final speed of the ball. You will get the same answer in these two descriptions.
I bring up the issue of time interval here because it will be difficult to discuss what happens after the ball hits the ground. More details are needed. In some instances, the time interval of interest is somehow implied.
————————–
Implications to teaching energy conversion
When students are first introduced (usually in high school) to the idea of energy and the conversions, oftentimes the teachers (me included) may not discuss the system of interest or time interval, because it will be too much cognitive load for students.
However, I think teachers need to find opportunities to discuss these two aspects properly because, in my opinion, one needs to be consistent in the approach of describing energy conversions.
For students
Embrace the fact that describing energy conversion is not going to be easy at the start. It will get easier as you continue to consider the two aspects when you describe energy conversion.
That’s all for now.
Chang Physics Class 