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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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# システムとオブジェクト (Systems and Objects)

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Bravo001 に公開 2021 年 01 月 14 日