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相关概念视频

Nuclear Fusion02:45

Nuclear Fusion

The process of converting very light nuclei into heavier nuclei is also accompanied by the conversion of mass into large amounts of energy, a process called fusion. The principal source of energy in the sun is a net fusion reaction in which four hydrogen nuclei fuse and ultimately produce one helium nucleus and two positrons.
A helium nucleus has a mass that is 0.7% less than that of four hydrogen nuclei; this lost mass is converted into energy during the fusion. This reaction produces about...
Conservation of Angular Momentum: Application01:18

Conservation of Angular Momentum: Application

A system's total angular momentum remains constant if the net external torque acting on the system is zero. Examples of such systems include a freely spinning bicycle tire that slows over time due to torque arising from friction, or the slowing of Earth's rotation over millions of years due to frictional forces exerted on tidal deformations. However in the absence of a net external torque, the angular momentum remains conserved. The conservation of angular momentum principle requires a change...
Kepler's First Law of Planetary Motion01:10

Kepler's First Law of Planetary Motion

In the early 17th century, German astronomer and mathematician Johannes Kepler postulated three laws for the motion of planets in the solar system. He formulated his first two laws based on the observations of his forebears, Nikolaus Copernicus and Tycho Brahe.
Polish astronomer Nikolaus Copernicus put forth a theory that stated a heliocentric model for the solar system. According to this heliocentric theory, all the planets, including Earth, orbit the Sun in circular orbits.
On the other hand,...
Tidal Forces01:06

Tidal Forces

The origin of Earth's ocean tides has been a subject of continuous investigation for over 2000 years. However, the work of Newton is considered to be the beginning of the proper understanding of the phenomenon. Ocean tides are the result of gravitational tidal forces. These same tidal forces are present in any astronomical body; they are responsible for the internal heat that creates the volcanic activity on Io, one of Jupiter's moons, and the breakup of stars that get too close to black holes.
Schwarzschild Radius and Event Horizon01:21

Schwarzschild Radius and Event Horizon

No object with a finite mass can travel faster than the speed of light in a vacuum. This fact has an interesting consequence in the domain of extremely high gravitational fields.
The minimum speed required to launch a projectile from the surface of an object to which it is gravitationally bound so that it eventually escapes the object’s gravitational field is called the escape velocity. The escape velocity is independent of the mass of the object. Merging the idea of escape velocity with the...
Radiation Pressure: Problem Solving01:09

Radiation Pressure: Problem Solving

The radiation pressure applied by an electromagnetic wave on a perfectly absorbing surface equals the energy density of the wave. The wave's momentum also gets transferred to the surface when an electromagnetic wave is entirely absorbed by it. The rate at which momentum is transmitted to an absorbing surface perpendicular to the propagation direction equals the force on the surface.
The average value of the rate of momentum transfer divided by the absorbing area represents the average force per...

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相关实验视频

Updated: Jun 24, 2026

Experimental System of Solar Adsorption Refrigeration with Concentrated Collector
07:18

Experimental System of Solar Adsorption Refrigeration with Concentrated Collector

Published on: October 18, 2017

太阳正在缩小吗?

I I Shapiro

    Science (New York, N.Y.)
    |April 4, 1980
    PubMed
    概括
    此摘要是机器生成的。

    从1736年到1973年分析了23次水星过境,发现没有太阳直径的显著变化. 基于地球的角径减小小于每世纪0.3弧秒,这与之前的发现相矛盾.

    科学领域:

    • 天文学 天文学
    • 太阳物理 太阳物理
    • 历史观察 历史观察

    背景情况:

    • 直接测量太阳直径的结果产生了相互矛盾的结果.
    • 历史过境观测为评估太阳变异性提供了一种替代方法.

    研究的目的:

    • 为了研究太阳直径的长期变化,使用历史的水星过境数据.
    • 为了将结果与基于直接太阳观测的先前研究进行比较.

    主要方法:

    • 分析了1736年至1973年期间记录的23次水星过境.
    • 回归分析以确定从地球观测到的太阳角径变化速率.

    主要成果:

    • 没有检测到太阳直径的统计学意义上的变化.
    • 计算出角径每世纪减少不到0.3弧秒 (90%的置信限).
    • 这一结果与埃迪的发现相矛盾,即每世纪减少2弧秒.

    结论:

    • 历史上的水星过境数据表明,太阳的直径在观测期间保持稳定.
    • 这些发现挑战了以往基于直接测量的太阳直径显著收缩的说法.

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