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  • Hi. It's Mr. Andersen and this is AP Physics essentials video 1. It is on objects and systems.

  • If we were to take something like a tennis ball, could you figure out if it is an object

  • or a system? The right answer is yes. It is both an object and a system. And it depends

  • on what question we are trying to answer. So if that tennis ball is a projectile and

  • we are studying projectile motion, the internal structure of the tennis ball is not relevant

  • to the question we are trying to solve. We are just angling the shot of the tennis ball

  • and figuring out how far it goes. And so we can treat the tennis ball as a single object.

  • However, if we were to look at the synthetic rubber that makes up the tennis ball and how

  • its elasticity changes at different temperatures, then it is more important to treat that as

  • a system made up of a bunch of atoms and understanding how those atoms behave at different temperatures

  • would be relevant to the problem that we are trying to solve. And so a system is made up

  • of two or more objects that are connected together in some way. Now if the internal

  • structure of that overall system is not important we can just treat that whole system as an

  • object. And so what is an object? It is anything that has no internal structure or no relevant

  • internal structure. Now you might think as we look smaller and smaller in the world of

  • physics, as we get down to the level of atoms then everything must be an object. But what

  • scientists have discovered is if you breakdown atoms we get particles inside those small

  • particles. Those are electrons, protons and neutrons. Some of these are what are called

  • fundamental and some are not. So when an electron is a fundamental particle. What does that

  • mean? You can not breakdown an electron into other parts. At least no parts that we have

  • discovered yet. And so we treat it as a fundamental particle. A single object. Protons and neutrons

  • are actually made up of other fundamental particles. And so a proton is made up of two

  • up quarks and one down quark. And a neutron is made up of two down quarks and one up quark.

  • And depending on the question that we are trying to answer, these parts inside those

  • protons and neutrons made be relevant to the problem we are trying to solve. For example

  • the charge of a proton and a neutron comes from its constituent parts. And so in physics

  • we are going to study matter through space and time at all these different levels. And

  • so since we are studying matter it is important that we understand a base unit on what matter

  • is made up of. And when we are solving problems you have to understand what is an object and

  • what is a system. And so in other words this hot air balloon could be treated as an object

  • if we are looking at buoyancy but if we are studying the kinetic energy of the molecules

  • we have to treat it as a system. And so a system is a collection of different objects.

  • And so this atwood machine here is made of two weights and then it is made up of a pulley.

  • And so we can treat that as a system. And each of those weights is treated as an individual

  • object. But if we were to zoom in to that one weight that also can act as a system.

  • It is made up of a mostly iron atoms. And that iron atom in turn is made up of subatomic

  • particles, electrons, protons and neutrons. And if we break those protons and neutrons

  • apart we find quarks on the inside of that. And so understanding all of the parts, depending

  • on what question we are trying to answer may become important. So let's talk about charge

  • for example. And so here are three fundamental particles. We've got an up quark, a down quark

  • and and electron, which is also a fundamental particle. And so if we want to figure out

  • where the charge of a proton, for example, comes from we could look at the charge of

  • each of these quarks. The charge of an up quark is two-thirds the charge of an electron.

  • And so it is two-thirds positive charge. A down is negative one-third charge of an electron.

  • So if we want to figure out the overall charge of a proton, what do we do? Well let's add

  • up the the charge of those two up quarks. So that is two-thirds plus two-thirds or four-thirds.

  • And then we have to subtract the one-third of the down quark. And so if we subtract one-third

  • from four-thirds we get three-thirds or a positive 1 charge of a proton. If we were

  • looking at the neutron for example, that is one up, which is going to be 1 two-thirds

  • charge, positive charge minus 2 one-third down charges. And so that is why a neutron

  • has no charge. And so if we are trying to figure out the charge of these particles then

  • the fundamental particles become important. And so did you learn the difference between

  • a system and an object? Again it depends on what question we are trying to answer. Are

  • the parts of the tennis ball important? If so then we must treat it as a system. If not,

  • we can treat it as an object. And I hope that was helpful.

Hi. It's Mr. Andersen and this is AP Physics essentials video 1. It is on objects and systems.


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B1 中級

系統和對象 (Systems and Objects)

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    Bravo001 發佈於 2021 年 01 月 14 日