Shadowing Practice: Proportional reasoning with motion | AP Physics | Khan Academy - Learn English Speaking with Video

NASA is researching how to send humans to Mars by as early as 2030.

Now, this is a complex mission because you're traveling for millions of kilometers, and this will involve a lot of things like when you think about how much fuel you need,

how much oxygen we need, and then how much time we'll be spending over there, and how much radiation exposure humans will be having.

So many different things.

So how do we figure out what's the best approach?

Well, what we usually do is try to come up with different plans.

I can call it as mission A, mission B, mission C, and so on and so forth.

And then we try to create mathematical models for them and then compare the models to find the trade-offs.

And then eventually we finalize on one of them.

And so the goal of this video is to get a glimpse of how to create mathematical models and compare them.

Now, of course, we'll not do it for such a complex mission.

We'll simplify it.

So in our case, we'll assume that the spacecraft is traveling in a straight line because that's simple.

and we'll assume that it's traveling with a constant acceleration.

And lastly, we'll assume that the initial velocity is zero.

So for the first case, let's assume, let's call this mission A, let's assume that the acceleration is 10 meters per second squared,

which is very close to the acceleration due to gravity on Earth.

So the astronauts over there would be feeling pretty much at home.

And the spacecraft will travel some distance, let's call it delta X, in some time.

But now let's create a second plan, second mission.

in which, again, it's constant acceleration, initial velocity is zero, it's going to travel the same delta x, but in half the time.

Okay?

Now, our goal is to compare some of the mission parameters.

The first thing we could compare is the average velocity.

I mean, think about it, right?

If we want to go from here to here in half the time as over here, then clearly the average velocity over here would be higher than over here.

That makes intuitive sense.

But how much higher?

We want to compare that.

So to do that, we want to build a mathematical model for the average velocity and then compare them.

So let's do that.

We can try using the model for average velocity, which is delta x divided by t.

Let's see if this model is useful for us.

Well, first of all, we need to compare average velocity.

So it's good that that's there.

Okay, what about delta x?

We know the relationship between them.

They're exactly the same.

Delta x for mission B is the same as delta x for mission A.

We're considering the same delta x value.

So that's good.

And what about time?

We also know the relationship between them.

We know the time over here has to be half the time over here.

So time for B is half the time for A.

So since we know the relationship for this between these two and the relationship for this between these two, we can now use this equation,

use this model, to find the relationship between the average velocity between the two.

So this is going to be a useful model.

Now imagine if we had used a model for average velocity which involved acceleration.

Then that wouldn't be useful because we don't know the relationship between the accelerations.

Right?

We don't know what the acceleration over here is.

And so that's why it's important to pick the right models.

But we don't have to worry too much.

If you do pick a wrong model, we'll figure it out and we'll scratch it and we'll use another model.

Trial and error.

No big deal.

Okay?

Anyways, let's go ahead with this.

Let's write down the average velocity for mission B.

So we could say the average velocity for mission B would be delta x B divided by T B.

And the average velocity for mission A would be delta x A divided by T A.

We know delta x B and delta x A are the same.

It's just delta x.

So let's just call them as delta x.

And now to compare them, we can just divide the two equations.

And now we simplify.

The delta x divides out.

And so you get that equals 1 over Tb divided by 1 over Ta, which is Ta divided by Tb.

And now we know what Tb is.

The time for this is half the time for A.

We can plug that in.

So if you get half Ta, the Ta divides out and you end up with 2.

So look, this ratio ends up becoming 2.

So we can rearrange now and write

that the average velocity in mission B will be twice as much as the average velocity in mission a.

So to cover the same delta x in half the time, we need twice the average velocity, which is not super obvious because we're dealing with accelerated motion over here.

But now the interesting question is, could we have just looked at this equation without doing the math, figure this out?

And the answer is yes.

If you look at this model carefully, because delta x is the same for both, we can look at this and we can say, hey, average velocity is proportional to 1 over t,

or it's inversely proportional to t.

What this means is that if t changes by some factor, your average velocity will change by the reciprocal of that factor.

So if t becomes double, your average velocity will be reciprocal of that, half.

If t triples, the average velocity will be reciprocal of that, one by third.

In our case, the t became half, so the average velocity would be reciprocal of half, which is two.

And so immediately, just by looking at this, we could have said, hey, if the t becomes half, the average velocity would double.

Powerful, right?

All right, now let's see if we can compare something else.

Let's compare their accelerations.

What's the acceleration in mission B compared to mission A?

And again, for that, we need to use a model, this time a mathematical model that involves acceleration.

And one of such models is this.

This is the model for things that are moving with constant acceleration, right?

Again, let's look at stuff that we already know and then see if this is going to be useful for us.

So what do we know?

Well, first of all, since we have x0, which is the initial position, which is over here, we can just set that to 0.

And then the x, which is the final position, we can just call that as delta x.

So we know those two.

That's the same for both of them.

100

We also know the initial velocity, which is v0.

101

That is 0.

102

And we know the time over here needs to be half the time over there.

103

and finally we know the acceleration of the first one

104

so these are the things that are given to us

105

so the first thing we can do is plug in whatever zeros are to make our model slightly simpler

106

so if you do that this goes to zero this becomes delta x this goes to zero

107

so you'll get delta x equals half a t squared

108

and since i want to compare accelerations i will rearrange this to get the acceleration

109

so acceleration becomes two delta x divided by t squared and again let's see if this model is useful for us.

110

I want to compare acceleration, so that's great.

111

I need accelerations.

112

Do I know the relationship between delta x?

113

Yes, they are the same.

114

And do I know the relationship between time?

115

Yes, I do.

116

So I can use this to find the relationship between the accelerations.

117

It'll be a great idea to pause the video and see if you can try this on your own, very similar to what we did earlier.

118

All right, let's do this.

119

So we can write down the accelerations for both the missions.

120

So for b will be 2 times delta x b by db squared, and for a will be 2 delta x a by ta squared.

121

But we know delta x b and delta x a are exactly the same, so we can just get rid of a and b over there.

122

And we can divide the two, just like before, and then simplify.

123

So the 2 delta x and 2 delta x can divide out, leaving us with 1 over tb squared divided by 1 over T A squared.

124

And then we can rearrange this to get T A squared divided by T B squared.

125

And again, we can finally plug in what T B is.

126

T B is half T A.

127

If we plug that in over here, we get half T A, the whole squared.

128

Finally, T A squared divides out, leaving us with 1 divided by 1 over 4, which is 4.

129

So from this, I now know that the acceleration in mission B would be four times as the acceleration in mission A.

130

And so there we have it.

131

We need four times more acceleration here compared to over here.

132

Four times more is four times 10, that is 40.

133

And then we could say that, hey, 40 meters per second square is too much of an acceleration for humans to withstand.

134

That's not gonna be possible because we'll be traveling for a long time.

135

And so we could say that, no, we can't do that.

136

Even if we reach in half the time, that's not possible.

137

But anyways, just like before, could we have figured this out without doing the whole math?

138

and the answer is again yes again

139

if you look at the model carefully we see

140

that delta x is the same for both of them

141

which means acceleration will be proportional to one over t squared

142

or inversely proportional to t squared this means whatever factor t

143

changes by acceleration will change by the reciprocal of the square

144

so if t were to double acceleration would be the reciprocal half square, one fourth.

145

If t were to triple, acceleration would change by one over three, reciprocal square, one ninth.

146

But in our case, t became half.

147

So acceleration would be reciprocal two squared, four.

148

And there you have it.

149

This is a very powerful way of doing this.

150

Since we're having so much fun over here, we should try one more plan, mission C.

151

Here, we'll keep the same acceleration as before, but we'll allow the spacecraft to travel for 50% more time than before.

152

So same acceleration, but 50% more time, it's going to travel farther, right?

153

Now the question is how much farther?

154

So we know what to do by now.

155

Think about a mathematical model.

156

So again, since we're dealing with positions and accelerations, let's use the same model as before.

157

Write down what we know.

158

And at any moment, feel free to pause and try this on your own, okay?

159

But let's write down what we already know.

160

We know, I mean, we can set our x to be 0, the initial position to be 0, and the final position would just be x.

161

So we can just set x0 to be 0.

162

We also know the v0 initial velocity is 0.

163

So we know that.

164

We also know the accelerations are the same in this time.

165

So acceleration case A and case C is the same.

166

And we are allowing for 50% more time.

167

So we know that the time here is 50% more compared to time A, which means 1.5 times A.

168

So Tc must be 1.5 times T A.

169

And again, we can plug in zeros to simplify.

170

So this goes to 0, this goes to 0 and that gives us a simplified model x

171

which is the final position which we want that is the model

172

so let's see if it's useful okay we want x we want to compare that

173

so it's good that's there do we know the relationship between

174

the accelerations yes the same accelerations do we know the relationship between the time yes we do

175

so we know the relationship between time 1.5 times ta and

176

so we can figure this out again we can do the math

177

but we also know how to do this in a shortcut now, right?

178

So we can look at this model and we can say that, hey, since the acceleration is the same, since A is the same,

179

x is directly proportional to t squared.

180

So now whatever factor t changes by, x will change by the square of that factor.

181

You can see that's directly proportional this time, right?

182

So if t doubles, x will be, x will change by a factor of two squared.

183

If t triples, x will change by a factor of 3 squared.

184

In our case, t has become 1.5 times more.

185

So x will change by a factor of 1.5 squared.

186

So we can immediately write xc, the final position over here, okay?

187

That has to be 1.5 times square xa, the final position over here.

188

And if you simplify, 1.5 square is 2.25.

189

And so we immediately get the answer.

190

So xc would be 2.25 times xa, which is amazing if you think about it, right?

191

Just by 50% more time, you're allowing, it'll travel more than twice the distance compared to the first one.

192

And by the way, we can check the math and I'll not go through all the steps over here.

193

I'll just show you all the steps.

194

You can pause the video and you can just see.

195

If you divide it and do it the long way, you get the same answer.

196

But anyways, look at what this means.

197

This means that, you know, in the first case, if we allowed it to travel for one year

198

and let's say the distance it traveled was a billion kilometers.

199

In the second case, if we allow it 50% more time, which is one and a half years, it will travel 2.25 billion kilometers.

200

That's awesome.

201

So long story short, by using mathematical models, we can see how changing one variable affects the other variable.

202

This is super useful in comparing scenarios.

203

What's important is that you don't need actual values.

204

You just need the models and you can divide them out and you can figure out and you can compare, you know, whatever you want to compare.

205

And another way is you can do it without dividing.

206

We can do it without doing the math by just looking at the model, seeing what variables are the same for the both,

207

and then figuring out the proportionality relationship, the right proportionality relationship.

208

If you know the factor by which one variable changes, we can figure out the factor by which the other variable would change as well.

209

This is not just useful for kinematics.

210

It's useful for all physics.

What is the Shadowing Technique?

Shadowing is a science-backed language learning technique originally developed for professional interpreter training and popularized by polyglot Dr. Alexander Arguelles. The method is simple but powerful: you listen to native English audio and immediately repeat it out loud — like a shadow following the speaker with just a 1–2 second delay. Unlike passive listening or grammar drills, shadowing forces your brain and mouth muscles to simultaneously process and reproduce real speech patterns. Research shows it significantly improves pronunciation accuracy, intonation, rhythm, connected speech, listening comprehension, and speaking fluency — making it one of the most effective methods for IELTS Speaking preparation and real-world English communication.

Shadowing Practice: Proportional reasoning with motion | AP Physics | Khan Academy - Learn English Speaking with Video

About This Lesson

Key Vocabulary & Phrases

Practice Tips

What is the Shadowing Technique?