Back To CourseAP Physics 1: Exam Prep
12 chapters | 136 lessons
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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.
We talk a lot about systems in physics, but this tends to be a tough concept for many students. The idea of a system is simple, but the application of it can be somewhat tricky. Simply put, a system is two or more objects. But this is a bit vague, isn't it?
To make it even trickier, what we are really interested in understanding is an isolated system. Now, instead of just any old set of objects, we have the added stipulation that the total amount of energy in the system doesn't change. Another way of looking at it is to say that there is no external net force on the objects in the system, or that the objects in the system are isolated from external forces.
An external net force on a system comes from something outside of the system. In other words, it comes from something other than the objects within the system itself. If you need to get a stalled car moving, you probably won't get very far if you sit in the driver's seat and push from the inside. Instead, you get a bunch of people to apply an external force from the outside. In fact, you could get 5 of the strongest men in the world to push from the inside, but in that position, they're part of the system, so the force doesn't do much good there.
In order for the force to be external, the force has to be unbalanced, meaning that there is no other force that opposes it with the same magnitude. If you and your friend push on opposite ends of the stalled car, it's going to stay right where it is because the two forces are balanced - they're acting with the same magnitude but in opposing directions. They are working against each other so they essentially cancel out, making both of you part of the isolated system! It would be much wiser to have your friend help you push from the same end so that the force is unbalanced, taking you out of the isolated system with the car.
The car is a pretty straightforward example of an isolated system because it's easy to see how you wouldn't move it by pushing from the inside or by pushing from opposite sides. But objects in an isolated system aren't always clearly contained like this.
For example, a head-on collision between two cars could either be an isolated system or not, depending on the forces involved. If the head-on collision is between two vehicles traveling on a road that provides a large amount of friction, the cars are not part of an isolated system because there is an external force of friction at work there.
However, take friction out of the equation and the crashing cars become an isolated system. This is because the forces they exert on each other cancel out as they collide, and there are no external forces involved.
Billiard balls are part of an isolated system because as they collide, they too exert balanced forces on each other. When the cue ball hits another ball, the momentum from the first ball is transferred to the second ball, sending it rolling across the table. In the absence of friction from the table, the two balls are part of an isolated system because there are no external forces at work.
It might help to think about isolated systems in terms of your morning coffee. You may notice that your coffee stays hotter longer if it is in a covered mug than one without a lid. In the covered mug, the heat from the coffee is 'isolated' from the external environment. However, the heat (a form of energy) is free to escape into the air in the uncovered mug because the lid 'barrier' doesn't exist. Of course, some heat is also conducted through the mug itself (which is why you pick it up by the handle), but if under ideal conditions there was no heat loss through the mug, the covered one would resemble an isolated system since there would be no loss of heat from this container.
The people in the car, the billiard balls on the table, and the cars in the frictionless collision are all like coffee inside a closed mug. Sometimes the mug is easy to see, like with the stalled car. Other times though, the 'mug' is invisible, as is the case with the head-on collision and the billiard balls.
Like many things in physics, we describe ideal situations such as cars driving on frictionless roads. But here on Earth, the absence of such forces is unrealistic, so there really aren't any perfectly isolated systems. External net forces are almost always present, and like our coffee cup, some heat is lost.
The universe is considered the only real isolated system, because it acts like a big, humongous stalled car that contains everything inside of it. Try as you might, you can't escape from the universe. All of the energy and heat inside of it simply gets transferred, transformed, and recycled as it moves through different forms and objects.
However, we only take up a tiny, tiny fraction of a fraction of the universe, so it's easier to understand how an isolated system COULD exist on a more local scale, with some minor assumptions of course. And understanding these small-scale examples helps us understand how the everyday world around us works in a way that is relevant and meaningful to us on Earth.
Physics can be a confusing subject because it challenges us to look at the world around us differently. And sometimes, the things we see are not always what they appear to be because there are other things going on that we don't see.
An isolated system is one of these tricky concepts because this is a system where the total amount of energy doesn't change. We can't necessarily see energy, so we have to use other knowledge to help us understand what's going on.
Like hot coffee in a sealed mug, objects in isolated systems are isolated from external forces. So as long as the total energy of the system doesn't change and there are no external forces acting on the objects in the system, we can say that the system is isolated.
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Back To CourseAP Physics 1: Exam Prep
12 chapters | 136 lessons