ฝึกพูดภาษาอังกฤษด้วยเทคนิค Shadowing จากวิดีโอ: Newton's second law | Middle school physics | Khan Academy

According to Newton's first law, if the net force acting on an object is zero, the object's motion will not change.

This means, for example, if an object is at rest on a table

or say somewhere in intergalactic space where the net force is zero, then the object will continue to be at rest.

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.

But now, here's a question.

What if the net force acting on an object is not zero?

Well, then the object's motion will change.

So a net force will change the object's motion.

In other words, a net force causes acceleration.

But how exactly are they connected?

That's what we're going to find out in this video.

So let's begin.

So how do we do this?

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.

and then if we do enough trials maybe we can see a pattern

and maybe we can see the connection between them

but of course the big question is how do we measure net force acting on an object

and how do we measure the acceleration of it well let's look at it one by one

so first of all how do we measure a net force well a cool equipment

that we can use to measure forces is a spring balance

and the way the spring balance works is

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.

And the number, the reading over here tells you exactly with how much force the spring is pulling back on you.

So in this example, the spring is pulling on my hand with a force of exactly two Newtons.

If you had pushed it from this end, then the spring is pushing with a force of exactly two Newtons.

And now since I know with exactly how much force it is pushing, I can use this to push on our objects

and look i will now know exactly with what force we are pushing on the object pretty cool isn't it

but wait a second

that is only one force remember our goal is to figure out the net force how do we do

that how do we figure out how much net force is acting on an object well let's see

if you were to keep these objects on a table then there are forces in the vertical like gravity

and normal force but they get balanced they are balanced isn't it

so we don't have to worry about those forces we only have to worry about the forces in the horizontal.

So one of the forces in the horizontal is our spring force, which we know.

But what about the other forces?

The other forces are the force of friction and air resistance.

We can minimize friction drastically by using an air hockey table, right?

And if the velocities are not too high, then even the air resistance is very minimal.

So look, in the horizontal, there's only one effective force, and that is the spring force.

So that itself becomes the net force.

Amazing, right?

Okay, so that's how we can measure the net force.

What about the acceleration?

How do we measure that?

Well, for that, we can use a motion sensor, which will periodically monitor the velocity, and it will give you a velocity time graph.

And by analyzing the graph, we can measure the acceleration.

Okay, so with that, we have everything needed.

We can go ahead and plan our experiment.

But how do we do that?

Well, one of the best ways to do that is to think about the variables involved.

So what are the variables involved over here?

Well, we have the net force, Since this is the variable that we can change independently, we call this the independent variable.

Then of course there is the acceleration that we're going to measure.

This acceleration is dependent on the net force, right?

So we call this the dependent variable.

So now for our experiment, we can push this object with different net forces.

Say we'll do three trials.

And then for each of them, we'll figure out what the acceleration is.

And by measuring the acceleration, maybe we can make a connection between the net force and the acceleration, isn't it?

Okay, that sounds like a plan, but there's another variable over here, the mass.

What do we do about that?

Should we also change that?

Well, here's an important thing about doing experiments.

You should always change just one variable at a time.

Since we're already changing the net force, this particular variable, we want to make sure the mass is a constant.

So we're going to use the same value of the mass everywhere.

And therefore, we call this the control variable because we're not changing that.

It's going to be a constant.

So what mass can we use?

Well, we can use any one.

Let's just use 1 kilogram mass.

So we now have our plan.

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.

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,

and we will also keep that contact for about a second.

So we'll put the force for about a second.

We'll try to make sure that this stays at two newtons, so the force is a constant, okay?

So let's do that.

And there we have it.

Once we let go of the contact, the object stops accelerating, and it continues to move in the straight line with the constant velocity.

So let's look at what our motion sensor has detected, and let's see what the velocity time graph looks like.

There we have it.

Let's see if that graph makes sense.

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.

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.

But how much is the acceleration?

We have to look at this.

And you can see the object's velocity increased from zero to two meters per second in one second.

So that is the acceleration, zero to two meters per second in one second.

All right, we're gonna do second trial, But this time we're gonna put four newtons of force.

And again, we'll push it for about one second and let it go.

All right, here we go.

Boom.

What do we notice?

Well, we notice that the object traveled much faster.

So let's see what the velocity time graph looks like.

What does our motion sensor give us?

Now the velocity time graph looks like this.

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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ฝึกพูดภาษาอังกฤษด้วยเทคนิค Shadowing จากวิดีโอ: Newton's second law | Middle school physics | Khan Academy

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