跟讀練習: Newton's second law | Middle school physics | Khan Academy - 通過影片學習英語口語
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According to Newton's first law, if the net force acting on an object is zero, the object's motion will not change.
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This means, for example, if an object is at rest on a table
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or say somewhere in intergalactic space where the net force is zero, then the object will continue to be at rest.
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On the other hand, if the object was moving, and again, if the net force on it was zero, then it will continue to move with that exact same velocity forever.
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But now, here's a question.
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What if the net force acting on an object is not zero?
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Well, then the object's motion will change.
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So a net force will change the object's motion.
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In other words, a net force causes acceleration.
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But how exactly are they connected?
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That's what we're going to find out in this video.
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So let's begin.
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So how do we do this?
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Well, we take a bunch of masses, apply a net force and measure that, and then measure how much acceleration they get because of the net force.
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and then if we do enough trials maybe we can see a pattern
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and maybe we can see the connection between them
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but of course the big question is how do we measure net force acting on an object
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and how do we measure the acceleration of it well let's look at it one by one
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so first of all how do we measure a net force well a cool equipment
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that we can use to measure forces is a spring balance
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and the way the spring balance works is
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that there's a spring over here so if you pull on it say from this side, then the spring is compressed and therefore it pulls back.
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And the number, the reading over here tells you exactly with how much force the spring is pulling back on you.
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So in this example, the spring is pulling on my hand with a force of exactly two Newtons.
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If you had pushed it from this end, then the spring is pushing with a force of exactly two Newtons.
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And now since I know with exactly how much force it is pushing, I can use this to push on our objects
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and look i will now know exactly with what force we are pushing on the object pretty cool isn't it
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but wait a second
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that is only one force remember our goal is to figure out the net force how do we do
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that how do we figure out how much net force is acting on an object well let's see
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if you were to keep these objects on a table then there are forces in the vertical like gravity
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and normal force but they get balanced they are balanced isn't it
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so we don't have to worry about those forces we only have to worry about the forces in the horizontal.
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So one of the forces in the horizontal is our spring force, which we know.
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But what about the other forces?
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The other forces are the force of friction and air resistance.
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We can minimize friction drastically by using an air hockey table, right?
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And if the velocities are not too high, then even the air resistance is very minimal.
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So look, in the horizontal, there's only one effective force, and that is the spring force.
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So that itself becomes the net force.
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Amazing, right?
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Okay, so that's how we can measure the net force.
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What about the acceleration?
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How do we measure that?
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Well, for that, we can use a motion sensor, which will periodically monitor the velocity, and it will give you a velocity time graph.
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And by analyzing the graph, we can measure the acceleration.
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Okay, so with that, we have everything needed.
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We can go ahead and plan our experiment.
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But how do we do that?
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Well, one of the best ways to do that is to think about the variables involved.
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So what are the variables involved over here?
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Well, we have the net force, Since this is the variable that we can change independently, we call this the independent variable.
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Then of course there is the acceleration that we're going to measure.
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This acceleration is dependent on the net force, right?
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So we call this the dependent variable.
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So now for our experiment, we can push this object with different net forces.
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Say we'll do three trials.
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And then for each of them, we'll figure out what the acceleration is.
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And by measuring the acceleration, maybe we can make a connection between the net force and the acceleration, isn't it?
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Okay, that sounds like a plan, but there's another variable over here, the mass.
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What do we do about that?
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Should we also change that?
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Well, here's an important thing about doing experiments.
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You should always change just one variable at a time.
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Since we're already changing the net force, this particular variable, we want to make sure the mass is a constant.
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So we're going to use the same value of the mass everywhere.
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And therefore, we call this the control variable because we're not changing that.
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It's going to be a constant.
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So what mass can we use?
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Well, we can use any one.
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Let's just use 1 kilogram mass.
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So we now have our plan.
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We're going to push the 1 kilogram object with different forces, and we're going to measure what the acceleration is going to be.
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All right, so we start by pushing with 2 newtons of force on a 1 kilogram object, and once we let go of this, the object will start accelerating,
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and we will also keep that contact for about a second.
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So we'll put the force for about a second.
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We'll try to make sure that this stays at two newtons, so the force is a constant, okay?
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So let's do that.
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And there we have it.
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Once we let go of the contact, the object stops accelerating, and it continues to move in the straight line with the constant velocity.
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So let's look at what our motion sensor has detected, and let's see what the velocity time graph looks like.
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There we have it.
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Let's see if that graph makes sense.
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For the first one second, the object was accelerating because there was a net force acting on it, and you can see the velocity is increasing.
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After the one second, the contact is lost and the object moves with the constant velocity and that's why look, the velocity stays put.
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But how much is the acceleration?
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We have to look at this.
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And you can see the object's velocity increased from zero to two meters per second in one second.
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So that is the acceleration, zero to two meters per second in one second.
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All right, we're gonna do second trial, But this time we're gonna put four newtons of force.
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And again, we'll push it for about one second and let it go.
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All right, here we go.
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Boom.
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What do we notice?
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Well, we notice that the object traveled much faster.
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So let's see what the velocity time graph looks like.
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What does our motion sensor give us?
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Now the velocity time graph looks like this.
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Ooh, what is it saying?
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Well, it's saying now in the first second while the object was accelerating, when the net force was acting,
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the velocity increased from zero to four meters per second.
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And then the object continues to move with a constant velocity of four meters per second because the contact was lost.
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So now the acceleration is zero to four meters per second in one second.
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The acceleration became higher when the net force became higher.
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Does this make intuitive sense?
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I think yes.
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It makes intuitive sense that when you apply a larger net force, the object accelerates larger.
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And we are also experimentally seeing that.
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That's amazing.
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Okay, if we did one more trial, we get very similar results.
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And this time, our motion sensor gives us something like this.
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And notice, we get even higher acceleration.
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This time the acceleration is, in one second, it goes from zero to five meters per second.
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So the acceleration has further increased.
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So what is the connection that we see?
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We see for a higher net force, more than net force, more is the acceleration provided the mass stays the constant.
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We should always be careful about the control variable that we have used.
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Amazing, isn't it?
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But now this brings up the last question.
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What if you want to vary the mass?
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We want to see the effect of what happens when the mass changes.
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Well, we should always make sure only one variable is changing.
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So if you want to vary the mass, we have to keep this variable the same.
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So we'll repeat this experiment.
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This time we'll keep the net force the same.
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Let's keep the net force, say, 4 newtons.
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And let's vary the mass.
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Let's make the mass 1 kilogram.
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Then let's repeat the experiment with 2 kilograms and maybe with 4 kilograms.
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All right, first up, we'll put 4 newtons for 1 kilogram.
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We actually already did this, but let's do it one more time.
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We'll push it for one second, let it go.
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And what does the motion sensor give us?
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Well, it gives us an acceleration of zero to four meters per second in one second.
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Okay, let's repeat this.
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Four newtons, but this time for two kilogram object.
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So again, push it for about a second, let it go.
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What does the motion sensor give us?
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The acceleration is smaller.
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Can you see that?
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It went from zero to two meters per second in one second.
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Does that make sense?
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I think yes.
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I mean, if you were to push a heavier object with the same force, you intuitively expect it to accelerate lesser.
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And that's exactly what we get.
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So as the mass increased, the acceleration reduced.
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So let's do one last trial.
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This time the mass is 4 kilograms.
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Again, we push it with the same force, 4 newtons for about a second.
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We can already see the acceleration was much lower.
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What does the motion sensor give us?
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Well, it gives us much lower acceleration.
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This time, it went from 0 to 1 meters per second in one second.
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So what do we notice?
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We notice that for a given force, if the net force is the same, if the mass increases, the acceleration reduces.
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So more mass gives you less acceleration for the exact same net force.
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This is the essence of what we call the Newton's Second Law.
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Of course we could be more quantitative, but I think this captures the essence beautifully.
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Newton's second law is one of the most important laws in physics
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because it allows us to predict the motion provided we know the net force acting on the object.
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為什麼要通過這個影片練習口語？
本影片深入探討了牛頓第二定律，適合英語學習者通過自然對話練習他們的口語能力。通過觀看和模仿影片中的講解者，您可以在輕鬆愉快的環境中提升自己的發音和語調，增強自信心。使用 shadowspeaks 技術，即影子跟讀，能有效提高您的口語流利度，使您能夠自然地表達複雜的物理概念，適應不同的交流場合。
語法與表達方式分析
在這段影片中，講解者使用了幾種重要的語法結構和表達方式，幫助學習者更好地理解內容：
- 條件句: “如果淨力作用於物體是零，那麼物體的運動將不會改變。”這一結構讓學習者瞭解如何表達假設情況。
- 被動語態: “力被平衡”有效地傳達了物體在不同力作用下的狀態。掌握被動語態對於複雜句子的構造至關重要。
- 疑問句: “那麼淨力是如何作用於物體的？”通過提出疑問，鼓勵觀眾思考並參與討論，增強互動性。
使用類似的句型練習，您可以在 看YouTube學英語 的過程中，更加靈活地使用英語口語。
常見發音陷阱
在影片中，講解者使用了一些特定的詞彙和術語，可能對很多學習者構成發音挑戰。例如：
- Acceleration（加速度）：注意這個詞的重音和音節劃分。
- Force（力）：在快速語速下，可能會發生不清晰的現象，需反覆練習以確保發音準確。
在練習這些詞彙時，可以藉助 shadow speech 方法，模仿講解者的發音。通過反覆的跟讀，您將能夠在實際交流中更自信地使用這些術語。
影片中的語法
說話人最常用的結構，並附上影片中的原話：
| 結構 | 影片中的用法 |
|---|---|
| 被動語態 be + 過去分詞 — 強調發生了什麼，而不是誰做的 | is compressed · are balanced · is lost |
| 用 “should” 提建議 should + 動詞 — 表示應該做的事 | Should we also change · should always change · should always be |
| 過去進行時 was/were + -ing — 過去某一時刻正在進行的動作 | was moving · was accelerating |
| 最高階 the -est / the most … — 一組中最高或最低的 | the best · the most important |
什麼是跟讀法？
跟讀法 (Shadowing) 是一種有科學依據的語言學習技巧，最初開發用於專業口譯員的培訓，並由多語言者Alexander Arguelles博士普及。這個方法簡單而強大：您在聽英語母語原聲的同時立即大聲重複——就像是一個延遲1-2秒緊跟說話者的影子。與被動聽力或語法練習不同，跟讀法強迫您的大腦和口腔肌肉同時處理並模模擬實的講話模式。研究表明它能顯著提高發音準確性，語調，節奏，連讀，聽力理解和口語流利度——使其成為雅思口語備考和真實英語交流最有效的方法之一。