Solve the particle judgment in physics, how to judge the particle point

When the focus of the study of the object is no longer on its own individual motion, but on its relative motion with the outside world, it can be used as a particle. For example, when studying the rotational motion of a table tennis ball, it is a non-mass point, and when studying its trajectory in the air (relative to the ground, the default reference system), it is a mass point. For example, the diving action of the water jumper is a non-quality point, and the study of its trajectory in the air is a mass point.

In general, it is more likely that translational objects, such as trains, can be regarded as particles, and whether it can be done depends on the impact of ignoring the size and length on the result. If you study the distance of the train from Beijing to Hangzhou, the train can be regarded as a particle, but if you study the time when the train passes through the bridge, it cannot be regarded as a particle

It's not that big things can't be regarded as particles, but small things can be particles. However, in general, the motion of two objects with a huge difference in size, small objects can generally be regarded as particles, such as when studying the motion of the earth around the sun, the earth can be regarded as particles.

Particle point, that is, for the problem studied, the large size of some objects can be ignored (relatively small), for example, the length of the train should be considered when studying the train entering the station, and it cannot be regarded as a particle point, and the speed of the train from Beijing to Shanghai can be regarded as a particle point. This is a relatively flexible model.

For example, when I study the force of a cup, I can treat it as a mass point (regardless of its appearance), and when I study the action of a diver, you can't treat it as a mass point, otherwise he is a point, what actions can you understand? That's about it. There is nothing else.

A particle can be defined as a point with mass, but it does not exist, it is an idealized model, that is, a scientific abstraction, when the size and shape of the substance have no effect on the research problem, the substance can be regarded as a substance.

Basis for judgment: (1) Whether to study the internal changes of the object itself, if so, it is not regarded as a mass point (such as gymnasts, divers, celestial rotation, cars or boats that study the movement of people or objects on a car, boat, and other similar situations, etc.). 2) Whether the shape, size, and motion of the object will have an impact on the research process and calculation results.

If there is a significant impact, it is not considered as a particle (such as a train crossing a tunnel, a truck crossing a bridge, etc.). If the internal motion of the object is not studied, the shape and size of the object are too small and negligible compared with the whole motion process, and its motion state does not affect the research process and calculation results, it can be regarded as a particle. For example:

The earth revolves around the sun, and the earth can be regarded as a mass point; The satellite orbits the earth, the satellite can be regarded as a particle, and the earth cannot be regarded as a particle.

It is not that a very large object must not be considered a particle, nor is it a very small object that can be considered a particle. )

If you study the universe, you can look at the Milky Way as a particle, if you study the Milky Way, you can look at the earth as a particle, if you study the distance on the ground, you can look at the city as a particle, you can see everything as a particle, if you want to, you can see everything as a particle, because the scope of your study contains an infinite number of particles.

The judgment of the particle is as follows:

Particle Definition:

A particle is a geometric point that only has the mass obtained by simplifying the object, regardless of the size and shape. When studying the motion of an object, if its size is very small within the scope of the study, or if the object is moving in a flat band (without rotating), it can be regarded as a mass concentrated at one point, called a particle, regardless of its size and shape.

A particle does not have to be a very small object, as long as the shape and size of the object are irrelevant or secondary factors in the problem being studied, i.e., the shape and size of the object have a very small effect on the problem under study, an object can be considered as a particle. It focuses on the study of the influence of objects on the system during motion and force, ignoring some complex but unrelated factors.

When studying the problem, the particle point is used instead of the object, and the difference in the motion state between the points on the object can be ignored. It is a scientifically abstracted concept in mechanics and an ideal model in physics.

Particle Determination:

To treat an object as a particle depends on the nature of the problem under study, not the object itself. Therefore, to be able to consider an object as a particle, one of the following conditions must be met:

When the size of the object is extremely small compared to the other distances in the problem under study. The motion of all parts of an object is the same, and the motion of any point of it can represent the motion of the whole object.

Under idealized conditions, all points on an object are in the same motion, and it can be regarded as a particle. The size and shape of the object have little impact on the research question and can be seen as a particle. A rotating object can also be regarded as a particle as long as its rotation is not studied and in accordance with Article 2.

Examples:

When testing a classmate's running speed and hunger, you can see this classmate as a quality point, but observing him doing radio exercises can be regarded as a quality point, because it is related to the movements of various parts of his body.

When the earth revolves around the sun, the earth can be regarded as a particle, because the diameter of the earth is much smaller than the distance between the earth and the sun, which is equivalent to a small dot moving around the big sphere of the sun, but when studying the rotation of the earth, the earth cannot be regarded as a particle.

1.Its size is small relative to the size of other objects.

2.Its size is small relative to the distance between the two.

3.Its shape is very regular, like a sphere, and it can also be reduced to a particle when the research group is scattered and carefully studied.

In short, the particle is an ideal model abstracted for the convenience of studying the collapse of objects.

For example, the earth revolves around the sun, at this time, because the size of the earth is small relative to the sun and the distance between the two, its own size has little influence on the sun, it can be regarded as a particle, but considering its rotation, its influence can not be ignored.

There are two types of moving objects that can be considered as particles:

1. The shape and size of the moving object are negligible compared with the problem it is studied, such as studying the revolution of the earth around the sun, and the earth can be used as a particle.

2. For a translational object, because the motion of each point on the object is the same, one point can be used to represent the motion of the whole object.

A particle is a point that is used to replace an object with mass regardless of shape and size, and is an ideal model that does not actually exist.

Under idealized conditions, the conditions are as follows: the motion of all points on the object is the same, which can be regarded as mass points; The size and shape of the object have little impact on the researcher's problem, and can be regarded as a particle.

You should be preparing! I'll give you a definition first, and then I'll give you some examples of what can and can't be considered as a particle, and then I'll summarize it at the end.

Definition: If the size and shape of the object itself have no or little effect on the study of its motion, we can replace the whole object with a point with mass, and this point with the same mass as the object is called a particle point that is used to replace the whole object.

One: Examples that can be regarded as quality points are as follows:

It can be seen as a particle:

Revolution of the Earth.

The rotation of the artificial satellite.

The running time of a 10,000-meter athlete.

A car passes through the Yangtze River Bridge in Nanjing.

Study the trajectory of table tennis.

Examples that cannot be seen as qualitative points, are given and explained as follows:

1.Train at the time of crossing the bridge:

The train crossing the bridge cannot be regarded as a quality point, but when looking for problems such as the time of the train crossing the bridge, it cannot be regarded as a quality point, because the length of the train and the bridge body are comparable and in an order of magnitude, so it cannot be regarded as a quality point. But if the train is long and the bridge is short, or the train is short and the bridge is long, then it can be regarded as a particle. This is very flexible, and the key depends on the specific problem.

2.Synchronized swimming, athletes at the time of diving:

Because the referee scores them according to their posture and movements, such as jumps, footwork, spins, etc., they must ignore these and treat the athlete as a point of mass, so the athlete cannot be regarded as a point of quality.

3.When the Earth rotates:

If the Earth's rotation is regarded as a particle, then what else can be studied? There is no rotation of the particle. Therefore, it cannot be regarded as a particle (revolution can be, rotation can't, so it depends on the specific research problem).

Induction and Difficulty:

If the size and shape of the object itself have no or little effect on the study of its motion, we can replace the whole object with a point with mass, which is used to replace the whole object with the same mass as the object, which is called a particle.

A particle is a point that has mass but no volume or shape. In general, if the size of the object is small or has little influence on the object of study, the object can be regarded as a particle.

Particle is a geometric point that only has mass and does not take into account size and shape obtained by simplifying the object, and is the most basic model commonly used in classical mechanics. Regardless of its size and shape, the displacement, velocity, and acceleration of any point in the body of an object in translational motion (see mechanical motion) are the same, which can be summarized by the motion of the point of its center of mass, which can be regarded as the motion of the particle. In the revolution of the earth around the sun, the displacement, velocity and acceleration of any point in the sphere to the sun are slightly different, but the radius of the earth is much smaller than the distance between the earth and the sun, and the above difference is also far less than the displacement[1], velocity and acceleration of the center of the earth.

Not necessarily, but also to consider the nature of the object's motion, whether its motion has nothing to do with the body, such as rotational motion, can not be regarded as a particle.