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국제바칼로레이트 · IB Diploma · Physics · SL 주제 13 3:51 영어 내레이션 · 영어 + 중국어 자막 burned-in
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An orbiting satellite is continuously falling while moving sideways.
轨道卫星在侧向运动的同时持续下落。
Being in orbit does not require gravity to vanish.
处于轨道状态并不要求引力消失。
Gravitational field strength is force per unit mass.
引力场强度是单位质量所受的力。
A gravitational force can provide the centripetal force for a circular orbit.
万有引力可以提供圆轨道所需的向心力。
Field and potential describe different quantities.
场和势描述的是不同的物理量。
Gravitational field: A description of gravitational force per unit mass.
引力场：单位质量所受引力的描述。
Centripetal force: Net force toward the centre of a curved path.
向心力：指向弯曲路径中心的合力。
For a point mass or outside a spherical mass, field strength follows an inverse-square distance dependence.
对于质点或球体外部，场强遵循与距离平方成反比的规律。
Use distance from the centre, not height above the surface alone.
请使用到中心的距离，而不仅仅是距表面的高度。
In the lab: State the circular-orbit approximation and ignore atmospheric drag only when justified.
在实验室中，仅在有依据时陈述圆轨道近似并忽略大气阻力。
Draw the force toward the central body and velocity tangential to the orbit.
画出指向中心天体的力以及沿轨道切线方向的速度。
Do not add an outward force merely because the path is circular.
不要因为路径是圆形就额外添加向外的力。
Known: field strength is 8 units at distance r from a spherical source.
已知：距球状源 r 处场强为 8 单位。
At 2r, g sub 2 divided by g sub 1 equals the quantity r over 2r squared, which equals 1 over 4.
在 2r 处，g₂/g₁ = (r/2r)² = 1/4。
Therefore, g sub 2 equals 8 divided by 4, which is 2 units.
因此，g₂ = 8/4 = 2 单位。
The gravitational force on a fixed test mass decreases in the same ratio.
固定试验质量所受的万有引力按相同比例减小。
Your turn: Field strength is 18 units at r.
请作答：在距离 r 处，场强为 18 个单位。
Find it at 3r for an inverse-square field.
对于平方反比场，求距离 3r 处的场强。
Pause the video and work it out.
请暂停视频，自己算一算。
Check: three times as far divides it by 9, so 18 becomes 2 units.
核对：距离变为 3 倍，强度除以 9，18 变为 2 个单位。
Weightlessness in an orbiting spacecraft means apparent weight is small in free fall, not that there is no gravitational field.
轨道飞行器中的失重意味着自由落体时视重很小，并非没有引力场。
Not true: Gravity is zero everywhere a spacecraft experiences apparent weightlessness.
这种说法不正确。 航天器经历失重感的区域，并非处处重力为零。
An orbiting spacecraft is in free fall: gravity is nearly as strong there as on the ground and supplies the centripetal force.
轨道运行的航天器处于自由落体状态：那里的重力强度几乎与地面相同，并提供向心力。
Crew and craft fall together, so nothing presses them down - that is apparent weightlessness, not zero gravity.
宇航员与航天器一同下落，因此没有力将它们压向下方——这是视重为零，而非零重力。
Gravitational field strength is force per unit mass.
引力场强度是单位质量所受的力。
For a point mass or outside a spherical mass, field strength follows an inverse-square distance dependence.
对于质点或球体外部，场强随距离的平方成反比变化。
Watch for: weightlessness in an orbiting spacecraft means apparent weight is small in free fall, not that there is no gravitational field.
请注意：轨道飞行器中的失重状态意味着自由落体时视重很小，并非没有引力场。
An inverse-square field has strength 36 units at distance r.
平方反比场在距离 r 处的强度为 36 个单位。
Find the strength at 3r.
求在 3r 处的强度。
Apparent weightlessness in orbit is not zero gravity.
轨道中的视重零并非真正的零重力。
The result is a field strength of 4 units.
结果是场强为4个单位。
Gravitational field strength follows an inverse-square law.
引力场强度遵循平方反比定律。
At twice the distance it is one quarter, and at three times the distance one ninth.
距离加倍时变为四分之一，距离为三倍时变为九分之一。
A spherical source produces field 12 units at r.
球体源在距离 r 处产生 12 单位的场。
At 2r the test mass is doubled.
在距离 2r 处，测试质量加倍。
Compare field and force with the original case.
请将此时的场和力与原情况作比较。
Field becomes 3 units, one quarter; force becomes one half because test mass doubles.
场强变为原来的四分之一，即三个单位。 力变为一半，因为测试质量加倍。
Field is independent of test mass; the inverse-square model applies outside the spherical source.
场强与测试质量无关。 平方反比模型适用于球体源外部。
Pause and check yourself. Explain each point on screen in your own words, then change one number in the last example and say which assumption must still hold.
暂停一下，自我检查：用自己的话解释屏幕上的每个要点，再改动最后一个例子中的一个数字，说明哪个假设仍然必须成立。