Back To CourseAP Physics 1: Exam Prep
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Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.
I don't know about you, but I need to eat a certain amount of food every day in order to feel good. We eat because the food we ingest provides us with energy as it is broken down inside our bodies.
This energy ultimately comes from the sun, and plants turn this into chemical energy that we then eat. Even if you just eat other animals, plants are still a part of your diet because somewhere down the food chain, a plant was eaten by something or someone.
In this sense, the energy that originally came from the sun gets converted and passed along through the biological system of Earth. What's amazing is that this amount of energy is always the same - there's no creation or destruction of energy as it gets passed through the system. When an animal eats a plant, it takes that energy into its body. If that animal gets eaten by another animal, the energy from the plant is transferred yet again. The second animal doesn't 'make' new energy; it just passes it along.
What happens when that animal dies? It puts that energy back into the ground, recycling it through the entire Earth system in yet another form. In fact, for any system, the total amount of energy never increases or decreases - it only changes form. This statement defines the law of conservation of energy and applies to all types of energy.
What exactly do we mean by a system? This is a situation when there are no external forces at work. A system can be a number of different things. On a small scale, a system might be something as simple as a swinging pendulum. On a very large scale, a system could be an exploding star. Regardless of how small, medium, or large the system is, its energy may change places or forms, but the total amount of energy stays the same.
Mechanical energy comes in two primary forms: potential energy, which is stored energy, and kinetic energy, which is energy of motion. Potential energy is named as such because it is energy that has the potential to do work.
Sitting at the top of a slide, standing at the top of a ski slope, and positioning yourself on the edge of a diving board are all examples of potential energy. You're not moving yet, but you have the potential to do so.
Once you take off down the slide, the ski slope, or over the edge of the diving board, that energy gets converted from potential to kinetic. This is because you are now in motion, and the energy has changed from potentially doing work to actually doing work.
In these scenarios, you are the system, and your energy gets converted from potential to kinetic as you set yourself in motion. If you come to a stop at the end of the slide, the hill, or in the water, the kinetic energy is converted back to potential energy because you have stopped moving. Regardless of where you are in these situations, the amount of energy at the top, through the movement, and at the bottom is the same - it just looks a bit different.
The conversion is not a sudden one, though, and we can see this if we look at a roller coaster car. As the car (the system) climbs to the top of the first hill, work is being done on it so there is a fair amount of kinetic energy as the car climbs the summit. Once at the top, there is a large amount of potential energy because the car has climbed so high. As the car sets off down the hill, that potential energy (potential to do work) becomes kinetic energy (energy of doing work). Throughout the fall, the potential energy becomes less and less, and the kinetic energy becomes more and more. At the bottom, the kinetic energy is very high, and the potential energy is very low.
After climbing the second summit, the kinetic energy is once again transformed back to potential energy - that is, until the car goes down the second hill, in which case that potential energy is converted back into kinetic energy. This goes on and on as the roller coaster travels along the track.
In the absence of resistance forces such as friction, the roller coaster car might continue on this way indefinitely because there would be no loss or gain of energy to the system. The same amount of potential and kinetic energy would transfer back and forth, and after a while, you'd probably wish you'd gotten on a different ride!
But the car does eventually come to a stop because there are in fact outside forces that act on systems. For example, the roller coaster car experiences friction as it moves along the track. Friction slows the car because energy from the car system is lost as heat as it rubs against the track. This means that as it travels along, there is less and less energy to keep it going up each successive summit.
This is the same reason your body can't sustain itself on one meal forever. The cells of your body are working hard all the time, and much of the energy you take in is lost as heat. This means you need a constant supply of energy because you are constantly 'burning' energy that drives cellular processes.
In this sense, we can see that one system can also be a part of other larger systems. The roller coaster car is its own system, but it's also part of the overall roller coaster system. Our bodies are a system, but they're also part of the system of Earth.
And, as long as there are no external forces at work on any individual system, the total amount of energy remains the same. For the roller coaster system, the car's energy and the energy lost as friction heat will always add up to the same amount of total energy. Your body on Earth takes in energy from other sources, but it also puts energy back out into the overall Earth system, keeping the total amount the same.
The law of conservation of energy states that for any system, energy cannot be created or destroyed; it can only change from one form to another or transfer from one object to another. Often the trickiest part about this is defining what a system is. In general, a system is a situation in which no external forces are at work. This could be something as small as a cell in your body or as large as Earth and all of its components.
This law applies to all forms of energy, including mechanical energy. Mechanical energy comes in two forms: potential energy, which is stored energy, and kinetic energy, which is energy of motion. Unless objects experience an outside force such as friction, the sum of potential and kinetic energy will always be the same no matter how much the object is moving or not moving.
If outside forces such as friction do act on the system, energy may be lost as heat. However, since a roller coaster car is also part of the overall roller coaster system, this energy 'loss' is really only an energy transfer to another object within the larger, more encompassing system.
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Back To CourseAP Physics 1: Exam Prep
12 chapters | 136 lessons
Next LessonInternal Energy of a System: Definition & Measurement