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Tổ chức Baccalaureate Quốc tế · IB Diploma · Physics · SL Chủ đề 2 3:58 Giọng đọc tiếng Anh · phụ đề tiếng Anh + 中文 được ghi trực tiếp
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A passenger continues moving when a vehicle brakes.
当车辆刹车时，乘客会继续向前运动。
The seat belt provides the force needed to change the passenger momentum 动量.
安全带提供了改变乘客动量所需的力。
Resultant force causes acceleration, not motion itself.
合力导致加速度，而非运动本身。
Momentum is mass multiplied by velocity.
动量等于质量乘以速度。
For an isolated system, total momentum is conserved even when kinetic energy is not.
对于孤立系统，即使动能不守恒，总动量也守恒。
Momentum equals mass multiplied by velocity.
动量等于质量乘以速度。
Resultant force is the vector sum of forces on an object.
合力是作用在物体上所有力的矢量和。
Impulse equals momentum change.
冲量等于动量的变化。
Increasing stopping time for the same momentum change reduces average force.
在相同的动量变化下，增加停止时间会降低平均作用力。
Identify external forces before applying momentum conservation.
应用动量守恒定律前，需先识别外力。
In the lab: Draw a free-body diagram containing only forces on the selected object.
在实验室中，绘制仅包含所选物体所受力的受力分析图。
For spring measurements, add loads in steps within the elastic range and measure extension from the unloaded position.
对于弹簧测量，需在弹性范围内分步加载荷，并从未加载位置开始测量伸长量。
Known: a 60 kg passenger slows from 10 to 0 metres per second in 0.50 s.
已知：一名质量为 60 kg 的乘客在 0.50 s 内从 10 m/s 减速至 0 m/s。
Delta p equals m times v minus u, which is 60 times 0 minus 10, equaling negative 600 kg metres per second.
动量变化 Δp = m(v-u) = 60(0-10) = -600 kg·m/s。
Average F equals Delta p divided by Delta t, which is negative 600 divided by 0.50, equaling negative 1,200 N.
平均力 F = Δp/Δt = -600/0.50 = -1,200 N。
The sign shows the force opposes the initial motion.
负号表示该力与初始运动方向相反。
Your turn: A 2 kg trolley travels at 3 metres per second.
轮到你了。 一辆质量为 2 kg 的小车以 3 m/s 的速度运动。
Find its momentum.
求它的动量。
Pause the video and work it out.
请暂停视频，自己算一算。
Check: 2 kilograms times 3 metres per second is 6 kilogram metres per second.
核对：2 千克 × 每秒 3 米，动量为 6 千克·米每秒。
Balanced forces do not require the object to be at rest.
平衡力并不要求物体处于静止状态。
Mass and weight have different units and meanings.
质量和重量具有不同的单位和含义。
Not true: An object moving at constant velocity must have a nonzero resultant force.
不正确。 以恒定速度运动的物体必须受到非零的合力作用。
By Newton's first law, constant velocity - including steady motion - means zero RESULTANT force.
根据牛顿第一定律，恒定速度包括匀速运动意味着合力为零。
Forces can be present, but they balance; a nonzero resultant would change the velocity.
可能存在力，但它们相互平衡；若合力非零，则会改变物体的速度。
Resultant force causes acceleration, not motion itself.
合力导致加速度，而非运动本身。
Impulse equals momentum change.
冲量等于动量的变化。
Watch for: balanced forces do not require the object to be at rest.
注意：受力平衡并不要求物体处于静止状态。
A 3 kg trolley travels at 7 metres per second and stops in 0.30 s.
一个质量为3千克的小车以每秒7米的速度运动，并在0.30秒内停止。
Find the momentum and the average force.
求动量和平均作用力。
Constant velocity needs zero resultant force.
匀速运动所需的合力为零。
The result is a momentum of 21 kilogram metres per second and an average force of 70 newtons.
结果是动量为21千克米每秒，平均力为70牛。
Every point has the same product of force and time, 600 newton seconds, because the momentum change is the same.
每个点的力与时间之积都是600牛秒，因为动量变化相同。
Doubling the stopping time halves the average force.
停止时间加倍，平均力减半。
A 2 kg trolley changes velocity from +3 to −1 m/s in 0.20 s.
一个两千克的小车在零点二零秒内速度从正三米每秒变为负一米每秒。
Find impulse and average force; explain why momentum change is not just change in speed.
求冲量和平均力，并解释为什么动量变化不仅仅是速率的变化。
The momentum change is 2 times minus 1 minus 3, which is minus 8 kilogram metres per second, and the average force is minus 40 newtons.
动量变化等于 2 乘以（−1 减 3），即 −8 千克·米每秒；平均作用力为 −40 牛。
The direction reverses although the speed falls from 3 to 1 metre per second.
虽然速率从每秒 3 米降到每秒 1 米，但运动方向反转了。
Using magnitudes loses the direction information.
只用大小会丢失方向信息。
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。