When I first learned about energy, I was introduced to different forms of energy, such as kinetic energy, gravitational potential energy, chemical energy, heat energy, light, etc. I also learned about one of the important conservation laws – the conservation of energy.I did not consider energy conservation from the systems point of view. Perhaps it was implied that the system is everything as described in the question.
All the while I have been thinking from the conversion point of view. Everything works quite well from my high school to my undergraduate years.
Only when I started my PhD in physics education research, I was introduced to thinking energy from the systems perspective. It was quite an eye-opener to me at that time. I was being introduced to this equation for the conservation of energy.
I find this approach particularly useful because I have another systematic way to approach physics problems related to energy.
My energy conversion approach has its utility too. Just that sometimes I may have to find reasons to account for why certain term needs to be on the left-hand side or right-hand side.
For example, a block is sliding down a rough inclined plane from rest. We want to find out the final speed of the block after it has traveled a distance D along the incline. The block has moved a vertical displacement of y downward.
From the energy conversion point of view, I know that if there is no friction, then the decrease in gravitational potential energy is the increase in kinetic energy. With friction, I expect that the change in kinetic energy to be less than before, so the equation looks like this
From the systems perspective, I can define the system in two ways:
System 1: block and inclined plane
The net force on the system is the gravitational force. The friction is internal to the system and it leads to an increase in the thermal energy when the block moves.
Initially, the total energy of system 1 is zero. Finally, the total energy of system 1 has kinetic energy and internal energy (work done by friction). The work done by net force (gravitational force) is positive. So the equation looks like
System 2: block, inclined plane, and earth
In this system, there is no external force, so the net work done is zero. The initial total energy of the system is the gravitational potential energy of the block.
The final total energy is the kinetic energy of the block, increase in internal energy of the system, and zero gravitational potential energy.
The equation looks like this:
Take a closer look at all 3 equations, they are actually the same, as it should be. If not, something is wrong with the energy analysis.
Takeaway messages:
1. You can pick the conversion perspective or the systems approach to solve conservation of energy problems. The answer is the same.
2. If you pick the systems approach, your choice of the system affects your energy analysis.
Let me know which one you prefer and the difficulties you encountered when you solve energy problems.
Chang Physics Class 