主题 5：力
AQA · GCSE · 物理 知识点 5 27:39 英语旁白 · 英中内嵌字幕
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Separate motion from change of motion.
区分运动与运动状态的改变。
A resultant force changes velocity; balanced forces can accompany motion.
合力改变速度；物体运动时也可以受平衡力。
Use a force diagram and state the motion before choosing an equation.
先画受力图并说明运动情况，再选择公式。
Equation-sheet support depends on the examination series.
公式表提供的内容取决于考试年份。
A vector needs a direction as well as a number.
矢量既需要数值，也需要方向。
Arrow length represents magnitude and the arrowhead gives direction.
箭头长度表示大小，箭头方向表示矢量方向。
Name the object receiving the force and the object exerting it.
说明哪个物体受力、哪个物体施力。
A third-law pair acts on two different objects.
牛顿第三定律中的两个力作用在不同物体上。
Contact forces include normal force, friction, tension and air resistance.
接触力包括支持力、摩擦力、拉力和空气阻力。
Gravity, electrostatic force and magnetic force can act between separated objects.
引力、静电力和磁力可以作用于相互分离的物体。
Weight is a force and is measured in newtons. Mass is measured in kilograms and does not change merely because the location changes.
重力是力，单位为牛顿；质量的单位为千克，不会仅因位置改变而改变。
Choose the local gravitational field strength.
使用当地的重力场强度。
Weight is directly proportional to mass at a fixed value of g.
当 g 不变时，重力与质量成正比。
Add forces in the same direction. Subtract opposing magnitudes and give the direction of the larger force.
同向力相加；反向力的大小相减，方向与较大的力相同。
Always include the direction of a non-zero vector answer.
非零矢量答案应包含方向。
Zero resultant means zero acceleration.
合力为零意味着加速度为零。
Thirty newtons north and forty newtons east, drawn to scale.
向北三十牛和向东四十牛，按比例画出。
Join the vectors head to tail; the resultant runs from the first tail to the final head.
把矢量首尾相接；合力从第一个矢量的尾指向最后一个矢量的头。
Measure its length and its angle on the diagram.
在图上量出它的长度和角度。
A mathematical magnitude alone does not replace the requested vector diagram.
仅给出计算结果不能代替题目要求的矢量图。
Draw clear arrows, state the scale and measure the direction.
画清箭头、说明比例尺，并测量方向。
A scale drawing gives both magnitude and direction.
比例图应给出大小和方向。
Check the resultant points into the quadrant defined by its components.
检查合力是否指向两个分力确定的象限。
Work is an energy transfer by a force through a displacement.
做功是力通过位移转移能量。
Select force and distance along the same direction.
应选取同一方向上的力和位移。
Work done equals force times distance, because the force is parallel to the displacement.
做功等于力乘以距离，因为力与位移方向平行。
Rearranged, F equals W over s.
变形得F等于W除以s。
Thirty-five joules divided by two point eight metres is twelve point five newtons.
三十五焦除以二点八米，等于十二点五牛。
One joule is one newton metre, and a force at right angles to the motion does no work along that path.
一焦等于一牛米；与运动方向垂直的力沿这条路径不做功。
Reduced resistance can increase the resultant without increasing the driving force.
阻力减小可以使合力增大，而不必增大推力。
Work and resultant force answer different questions.
做功与合力描述不同的问题。
Keep the actual two-mark demand separate from the added energy check.
把真题的两分要求与新增的能量问题分开。
Do not claim a fixed three-clause answer earns full credit on every question.
不能宣称固定三句话适用于所有题目的满分答案。
These are sheet 5.3 data.
这是练习 5.3 的数据。
The last measured proportional point is 6 N at 0.12 m, and the next point is non-linear.
最后一个满足正比关系的测量点为 6 N、0.12 m；下一点不再线性。
The exact limit lies between the measured loads.
准确的比例极限位于两个测量载荷之间。
Subtract the unloaded length to obtain extension.
用加载后的长度减去原长得到伸长量。
A graph of length can have a non-zero intercept even when extension is proportional to force.
即使伸长量与力成正比，长度图也可以有非零截距。
The unloaded spring already has a length, so zero force does not mean zero length.
弹簧不受力时本身就有长度，所以力为零并不意味着长度为零。
Within the proportional region, F equals k e, so k equals F over e.
在成正比的范围内，F等于ke，所以k等于F除以e。
Four point nought newtons divided by nought point nought six four metres is sixty-two point five, or about sixty-three, newtons per metre.
四点零牛除以零点零六四米，等于每米六十二点五牛，约六十三牛每米。
A larger spring constant means a stiffer spring.
弹簧常数越大，弹簧越硬。
Beyond the limit of proportionality, extension is no longer proportional to force.
超过比例极限后，伸长量就不再与力成正比。
Questions on this practical ask for the variables, a risk with its precaution, and ways to improve accuracy.
这个实验的考题要求写出变量、风险及对应措施，以及提高准确度的方法。
Explain why each change improves the measurement.
说明每项操作为什么能改善测量。
The extension is squared, so a unit-conversion error is also squared.
伸长量需要平方，因此单位换算错误也会被平方。
Use metres and check that the equation applies.
先换为米，并检查公式适用条件。
Work done stretching the spring is stored as elastic potential energy when the stated elastic model applies.
在题目给定的弹性模型适用时，拉伸弹簧所做的功储存为弹性势能。
The 300 N load at 2 m balances 150 N at 4 m.
2 米处的 300 N 与 4 米处的 150 N 平衡。
The diagram illustrates the force–distance trade, not conservation of a quantity called moment.
此图说明力与力臂的关系，不应把它称为力矩守恒。
The distance is perpendicular to the force line, not necessarily the length of the handle or the path moved.
力臂是支点到力的作用线的垂直距离，不一定是把手长度或运动路程。
State the moment in newton-metres.
力矩单位为牛顿米。
A longer lever can produce the same moment with a smaller force.
更长的力臂可以用更小的力产生相同的力矩。
Gear B turns because gear A exerts a force on it, producing a moment.
A 齿轮对 B 齿轮施力并产生力矩，所以 B 转动。
Look at the actual gear sizes and arrangement before stating its speed.
判断转速前应先看实际尺寸与传动结构。
Choose one pivot and use perpendicular distances consistently.
选择同一个支点，并一致使用垂直力臂。
The balanced moment relation is about that pivot.
平衡力矩关系应针对该支点。
The pressure difference comes from the different depths.
压强差来自深度差。
Upthrust is a force; compare it with weight and, for a moving object, drag.
浮力是力；应与重力比较，物体运动时还要考虑阻力。
Use pressure equals force over area for a normal force over an area. Use pressure equals h rho g, height times density times g, for the additional pressure due to a liquid column.
垂直于表面的力作用在一定面积上时，用压强等于力除以面积；液柱造成的附加压强，用压强等于高度乘以密度乘以 g。
Keep pressure due to the liquid separate from absolute pressure.
区分液柱造成的压强与绝对压强。
Convert areas to square metres before using p = F/A.
使用 p = F/A 前，应把面积换为平方米。
The words floating, sinking and accelerating do not all describe the same force balance.
漂浮、下沉与加速描述的受力情况不同。
Include drag when the object moves through the fluid.
物体在流体中运动时应考虑阻力。
Do not teach the universal rule that every sinking object has a downward resultant.
不能把所有下沉物体都说成合力向下。
Constant-velocity sinking is a counterexample.
匀速下沉就是反例。
Use one connected physical explanation.
给出连贯的物理解释。
Atmospheric pressure comes from air molecules colliding with surfaces and varies with altitude.
大气压强来自空气分子与表面的碰撞，并随高度变化。
Total distance follows the path. Displacement connects the start and finish with a direction.
路程沿实际路径计算；位移连接起点和终点，并包含方向。
The same trip can have non-zero average speed and zero average velocity.
同一次行程可以有非零平均速率，但平均速度为零。
Typical walking, running and cycling speeds are about 1.5, 3 and 6 m/s.
步行、跑步和骑车的典型速率约为 1.5、3 和 6 m/s。
On the positive velocity graph, triangle area is 80 m and rectangle area is 240 m.
正速度图中，三角形面积为 80 m，矩形面积为 240 m。
Its time scale matches the worked example.
时间轴与例题保持一致。
Use geometric areas only after identifying the time intervals.
先确定时间区间，再计算图形面积。
Signed area gives displacement; total distance adds the magnitudes if velocity changes sign.
有符号面积给出位移；若速度变号，求总路程时应把各面积的大小相加。
An instantaneous speed uses the tangent gradient at the chosen time.
瞬时速率应使用选定时刻的切线斜率。
A chord across an interval gives an average rate.
跨越一段时间的割线给出平均变化率。
Draw the tangent before choosing the gradient triangle.
先画切线，再选择斜率三角形。
State units from the axes.
单位由坐标轴确定。
Read the units of every given quantity.
检查每个已知量的单位。
The time must be converted before using the acceleration relation.
使用加速度公式前，必须先换算时间。
Two point five minutes times sixty is one hundred and fifty seconds.
二点五分钟乘以六十，等于一百五十秒。
Acceleration equals change in velocity over time, so v equals u plus a t.
加速度等于速度变化量除以时间，所以v等于u加at。
Nought plus nought point six four times one hundred and fifty gives ninety-six metres per second, downwards, in the direction of the jump.
零加零点六四乘以一百五十，得每秒九十六米，方向向下，与跳伞方向一致。
This is a mean acceleration, so do not assume it was constant at every instant.
这是平均加速度，不要以为每一时刻的加速度都相同。
Before terminal velocity, draw the drag arrow pointing upward and shorter than the weight.
达到末速度之前，空气阻力箭头向上，且短于重力箭头。
The numbers here come from a separate worked example on sheet 5.7.
图中数值来自练习 5.7 的另一道例题。
The answer needs connected reasoning.
答案需要连续的推理。
Opening the parachute first increases drag, causing an upward acceleration while the skydiver is still moving down.
开伞后阻力先增大，跳伞者仍向下运动，但加速度向上。
A third-law pair acts on different objects and cannot balance one another on a single free-body diagram.
第三定律中的力作用于不同物体，不会在同一物体的受力图中互相平衡。
Inertia is the tendency to maintain rest or uniform motion.
惯性是保持静止或匀速直线运动状态的性质。
Weight and resultant are different forces in this calculation.
此题中重力与合力不同。
Take downward as positive and subtract drag from weight.
取向下为正，合力等于重力减去空气阻力。
The drag is smaller than the weight, matching the downward acceleration and the free-body diagram.
空气阻力小于重力，与向下的加速度及受力图一致。
For the combined system, hanger weight provides the driving force when resistance is negligible or compensated.
把小车和悬挂砝码看作整体；忽略或补偿阻力时，悬挂物重力为驱动力。
String tension is a force on the trolley, not simply the hanger weight.
绳子对小车的拉力不能直接等同于悬挂物重力。
At fixed mass, acceleration is proportional to resultant driving force. At fixed force, acceleration is proportional to the reciprocal of total moving mass.
质量不变时，加速度与合外力成正比；合外力不变时，加速度与总运动质量的倒数成正比。
Distinguish variation between readings from a systematic offset.
区分重复测量的离散程度与系统偏差。
Repeating measurements does not automatically remove systematic error.
重复测量并不能自动消除系统误差。
The range is one point three nine minus one point three three, which is nought point nought six metres per second squared.
极差为一点三九减一点三三，即零点零六米每二次方秒。
The uncertainty is half the range: plus or minus nought point nought three metres per second squared.
不确定度是极差的一半：正负零点零三米每二次方秒。
Repeats reveal the spread, and a mean reduces random variation, but a miscalibrated timer could still shift every result.
重复测量能显示数据的分散程度，取平均值能减小随机误差，但计时器若校准不当，仍会使每个结果都产生偏移。
At 20 m/s with a 0.60 s reaction time, thinking distance is 12 m.
20 m/s、反应时间 0.60 s 时，反应距离为 12 m。
Adding the supplied 15 m braking distance gives a 27 m stop.
加上已知 15 m 制动距离，停车距离为 27 m。
Do not mix the driver’s reaction with the vehicle’s braking.
不要混淆驾驶员反应与车辆制动。
Tiredness, alcohol, drugs and distraction can lengthen reaction time; typical times are 0.2 to 0.9 seconds.
疲劳、酒精、药物和分心可延长反应时间；典型值为 0.2 至 0.9 秒。
The distance in the uniform-acceleration equation is the braking distance.
匀加速度公式中的路程为制动距离。
Thinking distance belongs to the earlier reaction stage.
反应距离属于制动前的阶段。
Use v squared minus u squared equals two a s, so a equals v squared minus u squared, over two s.
用v平方减u平方等于2as，所以a等于v平方减u平方，再除以2s。
Nought minus eighteen squared, over two times twenty-four, gives minus six point seven five metres per second squared.
零减十八的平方，除以二乘二十四，得负六点七五米每二次方秒。
Then F equals m a: one thousand four hundred kilograms times minus six point seven five gives minus nine thousand four hundred and fifty newtons.
再用F等于ma：一千四百千克乘以负六点七五，得负九千四百五十牛。
The force is nine thousand four hundred and fifty newtons, opposite to the motion; state your sign convention.
力的大小为九千四百五十牛，方向与运动方向相反；要说明正方向的规定。
Explain why the brakes heat up, then give two risks of a large deceleration.
先解释刹车为何发热，再给出大减速度的两个风险。
Do not claim that any one impact force is guaranteed safe.
不能把某个碰撞力称为一定安全。
Release without a countdown, keep the same hand and starting position, and repeat both conditions.
不要倒计时释放；保持同一只手和相同起始位置，并重复两种条件。
Use a supplied conversion scale or an appropriate falling-distance relation.
使用给定换算表或合适的自由落体关系。
Explain the purpose of controls.
说明控制变量的目的。
A consistent procedure helps separate the distraction effect from other differences.
一致的操作有助于把分心的影响与其他差异区分开。
Momentum is Higher Tier. Collision calculations and rate-of-change applications are physics-only parts.
动量属于高阶内容；碰撞计算和动量变化率应用属于仅物理部分。
Choose one positive direction and keep it throughout.
选择一个正方向并始终保持一致。
The combined mass moves with one velocity after the collision.
碰撞粘合后，总质量以共同速度运动。
Momentum conservation does not require kinetic energy to be conserved.
动量守恒并不要求动能也守恒。
Add signed momenta before equating the totals.
先把有符号动量相加，再列守恒关系。
The assumption concerns the combined system during the collision.
假设针对碰撞期间的两小车整体。
Subtract initial velocity from final velocity with their signs.
保留方向符号，用末速度减初速度。
A rebound is not a speed subtraction of twelve minus eight.
反弹不能只用十二减八来求速度变化。
The force calculated over the contact interval is a mean force.
接触时间内计算的是平均力。
The instantaneous force can vary during the impact.
碰撞过程中瞬时力可以变化。
Compare the same driver brought from the same velocity to rest.
比较同一驾驶员从相同速度停止的情况。
The momentum change is the same, but the time can differ.
动量变化相同，但停止时间可以不同。
Keep the three connected physical links clear: longer time, lower momentum-change rate, smaller force.
把三个物理环节连接清楚：时间更长、动量变化率更小、平均力更小。
A crumple zone extends the time and distance of the stop.
溃缩区延长停车时间和距离。
Use mean forces when the collision force varies.
碰撞力变化时，应用平均力进行比较。
A useful answer names the quantity, the model and the condition that makes the model valid.
有效答案应说明物理量、模型及模型成立的条件。
Return to the relevant sheet whenever you can recognise an equation but cannot explain why it applies.
如果只会认公式，却不能说明适用原因，应返回相应练习进行训练。