القوى وتأثيراتها
OxfordAQA · International GCSE · Physics الموضوع 1 6:54 سرد باللغة الإنجليزية · ترجمة مدمجة بالإنجليزية + الصينية
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النص الصوتي
Every push, pull and hold in the world is a force.
世界上每一次推、拉、托住都是力。
This topic organises them: contact and non-contact interactions, scalars and vectors, Newton's three laws and vehicle safety.
本主题把它们整理成：接触力与非接触力、标量与矢量、牛顿三大定律和车辆安全。
The Paper 2 extension adds momentum, terminal velocity, centre of mass and moments.
第二卷的拓展内容还包括动量、终端速度、质心和力矩。
Objects interact in two ways.
物体之间的相互作用有两类。
Non-contact forces act through a field: gravity, electrostatic and magnetic forces.
非接触力通过场起作用：重力、静电力和磁力。
Contact forces need touching surfaces: friction, air resistance, tension and the normal contact force.
接触力需要表面接触：摩擦力、空气阻力、张力和法向接触力。
Friction acts between two surfaces, opposes motion and can heat them.
摩擦力作用在两个表面之间，阻碍运动，并可能使它们发热。
A scalar has size only: thirty metres.
标量只有大小：三十米。
A vector has size and direction: thirty metres east.
矢量既有大小又有方向：向东三十米。
Draw a vector as an arrow, its length for the magnitude and its direction for the direction.
矢量画成箭头，长度表示大小，箭头方向表示方向。
Distance and speed are scalars; displacement, velocity, force and momentum are vectors.
路程和速率是标量；位移、速度、力和动量是矢量。
Weight is the force of gravity: W equals m times g.
重力是地球对物体的引力：W 等于 m 乘以 g。
On Earth g is about 9.8 newtons per kilogram, so a 5.0 kilogram bag weighs 5.0 times 9.8, which is 49 newtons.
在地球上 g 约为每千克 9.8 牛，所以 5.0 千克的包重 5.0 乘 9.8，即 49 牛。
Its mass is the same on the Moon; its weight is smaller, because g is smaller.
在月球上它的质量不变；重力变小，因为 g 变小了。
Hang masses on a spring and plot force against extension.
在弹簧下挂砝码，画出力与伸长量的关系图。
The points make a straight line through the origin, and its gradient is k, the spring constant: F equals k times e.
各点连成一条过原点的直线，斜率就是劲度系数 k：F 等于 k 乘以 e。
Past the limit of proportionality the line curves, and the spring may not return to its length.
超过比例极限后，图线弯曲，弹簧可能无法恢复原长。
A spring is 12.0 centimetres long, and 15.0 with a 3.0 newton load.
一根弹簧原长 12.0 厘米，挂 3.0 牛时长 15.0 厘米。
The extension is 3.0 centimetres, 0.030 metres, so k is 3.0 divided by 0.030, 100 newtons per metre.
伸长量是 3.0 厘米，即 0.030 米，所以 k 等于 3.0 除以 0.030，即每米 100 牛。
With 5.0 newtons the extension is 0.050 metres, so the spring is 17.0 centimetres long.
挂 5.0 牛时伸长 0.050 米，弹簧长 17.0 厘米。
A train speeds up steadily from rest to 20 metres per second in 40 seconds, then runs steadily for 20 seconds.
一列火车从静止匀加速，40 秒内达到每秒 20 米，然后匀速行驶 20 秒。
Acceleration is the gradient: 20 divided by 40, 0.50 metres per second squared.
加速度是斜率：20 除以 40，等于每二次方秒 0.50 米。
Distance is the area: the triangle gives 400 metres and the rectangle 400 metres, 800 in all.
距离是面积：三角形 400 米，长方形 400 米，共 800 米。
Newton's first law: with zero resultant force, an object at rest stays at rest and a moving object keeps its velocity.
牛顿第一定律：合力为零时，静止的物体保持静止，运动的物体保持速度不变。
The second law: F equals m times a, in the direction of the resultant force.
第二定律：F 等于 m 乘以 a，方向与合力相同。
The third law: two interacting objects exert equal and opposite forces on each other.
第三定律：两个相互作用的物体对彼此施加大小相等、方向相反的力。
A 1200 kilogram car has a driving force of 3000 newtons forwards and resistive forces of 600 newtons backwards.
一辆 1200 千克的汽车，向前的驱动力为 3000 牛，向后的阻力为 600 牛。
The resultant is 2400 newtons forwards, so the acceleration is 2400 divided by 1200, 2.0 metres per second squared.
合力为向前 2400 牛，所以加速度等于 2400 除以 1200，即每二次方秒 2.0 米。
At a steady speed later, resistance has grown to 3000 newtons.
之后匀速行驶时，阻力已增大到 3000 牛。
Two skaters push each other.
两名滑冰者互相推。
The force on A, from B, points one way; the force on B, from A, points the other way, and they are equal in size.
B 对 A 的力指向一边，A 对 B 的力指向另一边，大小相等。
They never cancel, because they act on different objects: skater A and skater B both move off.
它们永远不会抵消，因为作用在不同物体上：两名滑冰者都会滑开。
A boat's engine pushes east with 400 newtons and the wind north with 300.
船的发动机向东推 400 牛，风向北推 300 牛。
With 1 centimetre for 100 newtons, draw 4.0 centimetres east, then 3.0 centimetres north from its head.
用 1 厘米代表 100 牛：先向东画 4.0 厘米，再从箭头处向北画 3.0 厘米。
The resultant, start to end, measures 5.0 centimetres: 500 newtons, at 37 degrees north of east.
从起点到终点的合力长 5.0 厘米，即 500 牛，方向为东偏北 37 度。
Momentum p equals m times v, and in a closed system the total is conserved.
动量 p 等于 m 乘以 v，封闭系统中总动量守恒。
Two skaters, 60 and 40 kilograms, stand still and push apart, so the total is zero before and after.
两名滑冰者分别为 60 和 40 千克，静止后互推分开，所以前后总动量都为零。
The 40 kilogram skater has 120 kilogram metres per second, so the other moves at 2.0 metres per second the other way.
40 千克的一人动量为 120 千克米每秒，所以另一人以每秒 2.0 米向反方向运动。
Force equals change of momentum divided by time.
力等于动量变化除以时间。
A 0.40 kilogram football kicked to 25 metres per second gains 10 kilogram metres per second; in 0.010 seconds of contact the force is 1000 newtons.
0.40 千克的足球被踢到每秒 25 米，动量增加 10 千克米每秒；接触 0.010 秒，力为 1000 牛。
Make the contact ten times longer and the force is ten times smaller.
接触时间长十倍，力就小十倍。
Crumple zones and airbags do exactly this.
溃缩区和安全气囊正是这样保护乘客的。
Stopping distance is thinking distance plus braking distance.
停车距离等于反应距离加制动距离。
Thinking distance grows with reaction time: tiredness, distraction, drugs and alcohol.
反应距离随反应时间增加：疲劳、分心、药物和酒精。
Braking distance grows on wet or icy roads and with worn brakes or tyres.
路面湿滑或结冰、刹车或轮胎磨损时，制动距离变长。
Both grow with speed.
两者都随车速增加。
A skydiver accelerates until drag equals weight, then falls at a steady terminal velocity.
跳伞者不断加速，直到阻力等于重力，然后以稳定的终端速度下落。
When the parachute opens, drag is suddenly larger than weight, so she slows down, until drag and weight balance again at a much lower terminal velocity.
降落伞打开时，阻力突然大于重力，所以她减速，直到阻力与重力再次平衡，达到低得多的终端速度。
The centre of mass is where the mass can be thought concentrated. Its weight acts straight down from there.
质心是可以认为质量集中的点，重力从那里竖直向下作用。
While that line falls inside the base, the object stays upright.
只要这条线落在底面以内，物体就保持直立。
Tilt it until the line falls outside the base, and the moment of its weight topples it.
把它倾斜到这条线落在底面以外，重力的力矩就会使它倾倒。
A 300 newton child sits 2.0 metres to the left of the pivot: an anticlockwise moment of 600 newton metres.
一个 300 牛的孩子坐在支点左侧 2.0 米处：逆时针力矩为 600 牛米。
To balance, a 400 newton child needs a clockwise moment of 600, so sits 600 divided by 400, 1.5 metres from the pivot.
为了平衡，一个 400 牛的孩子需要 600 牛米的顺时针力矩，所以坐在离支点 600 除以 400，即 1.5 米处。
The heavier child sits closer.
较重的孩子坐得更近。
Four common mistakes. Using the spring's length instead of its extension. Using one force in F equals m a when several act. Adding perpendicular forces as numbers: 400 and 300 newtons make 500, not 700. And thinking a third-law pair cancels: the forces act on different objects.
四个常见错误：用弹簧长度代替伸长量；有多个力时只用一个力代入 F = ma；把相互垂直的力直接相加：400 牛和 300 牛的合力是 500 牛，不是 700 牛；以为第三定律的一对力会抵消：它们作用在不同物体上。
Check yourself.
自我检测。
A 900 kilogram car accelerates at 2 metres per second squared: the resultant force is 1800 newtons.
900 千克的汽车以每二次方秒 2 米加速：合力为 1800 牛。
Of speed, displacement, time and force, the vectors are displacement and force.
速率、位移、时间和力中，矢量是位移和力。
A 250 newton force 0.4 metres from a pivot has a moment of 100 newton metres.
距支点 0.4 米处的 250 牛力，力矩为 100 牛米。