Hi joseotillio25, thanks for the question. Since the problem says we're working with protons, that means the charge on each is the elementary charge of $1.6 \times 10^{-19} \textrm{ C}$.

Thanks for spotting that. Yes, you're quite right that there's a small error in the subscripts, and it should be $r_{12}^2 + r_{23}^2 = r_{13}^2$. I've made a note in the quick answer for other students.

I have also noticed that this answer is incorrect from the book's answer key section. The given information does not seem to add up to Giancoli's question #39, I believe this may be a completely different question. I am using the seventh edition book and I believe it is not the "Global Edition", although the other questions have been correct so far.

Hi grjoav, thanks for the comment. In the Global Edition, Chap. 7 #39 is about a baseball hitting a brick, whereas in the regular 7th Edition Chap. 7 #39 is about two cars colliding, as covered in the video. What is the difference you're finding between what's in your text versus what's covered in the video?

Hi jiwook.kim, thanks for the comment. At 2:58 in the video I noticed the error in the separation between slits that you pointed out and changed it to $0.62 \textrm{ mm}$, and the correct value was used in the later calculations so the final answer is still correct. Thanks for mentioning this anyways since if there was a mistake then it would be important to let other students know.

I think you went over this in previous videos, but does it matter which "loops" you choose? For instance, I chose the top loop and the bottom loop instead of the top loop and larger outer loop, but my values were slightly off. Should I have gotten the same answer? Thanks in advance for your help.

Hi ctlawson, it doesn't matter which loops you choose. Your goal with 'choosing loops' is to create a collection of equations on which you can then do algebra to figure out the unknowns. You need as many equations as you have unknowns. Each loop is an example of Conservation of Energy, in that the total gain and loss in potential around a closed loop must equal zero. You begin the loop at some point, travel along, and then return to where you started, but if in doing so you arrived with a different amount of energy than when you started, then some energy must have been created or destroyed along the way, which is a violation of Conservation of Energy. Anyhow, I say all this to try and be convincing that the choice of loop doesn't matter. The discrepancy between our answers could be due to intermediate rounding error, or check that you included the $1.0 \Omega$ resistors, or something like that.

At 6:09, you added 1.3235 and 6.8421 to get 8.1556 and then you added -1.4118 to -2.5263 to get 4.9381. Should you have gotten 3.9381(I3)=8.1656 instead?
Thanks in advance for your help.

Hi ctlawson, thanks for the question. At 6:09 two things were done at once: $-1.4118I_3 - 2.5263I_3 = -3.9381I_3$ as you say, but the second thing done is that this quantity was moved to the other side of the equation. There is the term $1I_3$ over there, so the result is $1I_3 + 3.91381I_3 = 4.91381I_3$ on the left side.

Hope that helps, and all the best with your studies,
Mr. Dychko

Hi thesouthportschool, the negative sign in $W= -qV$ that you're used to seeing is the work done by the electric field. Since, in this question, we're asked to find the work done by an external force, the external force is applying a force in the opposite direction to that of the electric field. For this reason the work done by the external force has a sign opposite to that of the electric field, which means $W_{ext} = qV$ with no negative sign.

Hi gregory.wallace, since the elbow is chosen as the pivot, we need to measure each 'lever arm' by starting at the elbow. $r_1$ is measured starting at the elbow.

It seems that there is a small typo in your answer for the location for the charge. In the video you say that the location is .37l, not .27l. (~4:10 in video)

Hi ceward, thanks for the comment. I'm pretty sure in this case the solution in the back of the book is not correct. I double checked the calculations in the video, and it all checks out. Let me know if you notice any error in the working.

Unless I am missing something, you have a silly math error here in comparing the value predicted from the gas law, 0.588, to the real value, 0.598. The difference is 0.01, not 0.02 as you wrote it.

Hi ctlawson, as part of our job in this question we need to break the electric fields into 'x' and 'y' components. It would be perfectly OK to use $60^\circ$. The difference is that you would have to think about which trig. function is the right one to use. For example, when I calculate $E_{Bx}$ using $\cos(30^\circ)$ you could instead use $\sin(60^\circ)$. It's just a matter of personal preference.

Hi jamesswaggernaut, darn, you're right. The process to solve it for part b) is exactly the same as what's shown for part a). Since the internal resistances are in series with the resistor in the same branch as the battery, the only difference compared with a) is to add one more ohm to $R_1$ and to $R_3$. I've made a note about this in the quick answer, and thanks for spotting it.

How do we know that E1 and E2 have the same magnitude? Is that always the case, even though one is closer and one has a stronger charge than the other?

Hi magiemay480, thanks for your question. E1 and E2 having equal magnitude is a special case for this question. The reason for it is that the electric field at point "P" is zero, and the only way for that to be possible is if E1 and E2 are of equal magnitude in opposite directions.

Hi mschick16, thanks for the question. I suppose it was unnecessary to take the absolute value of KE. You can safely ignore that part. It's just the way I was thinking about the problem in terms of magnitudes, rather than bothering with direction. Consider this different approach that I think you'll prefer in which I'm following the negative signs properly:

I think you'll agree that $\Delta KE = -\dfrac{1}{2}mv_i^2$, and the video shows that $\Delta KE = F_{net}d$, so this means $-\dfrac{1}{2}mv_i^2 = F_{net}d$. This means $F_{net}$ is negative, which we expect from the diagram since it's the friction force directed opposite to the direction of the initial velocity. $F_{net} = 0 - F_{fr}$ where I'm taking all the forces to the right (of which there are none, hence the zero), minus the forces to the left (just friction). Therefore $F_{net} = -F_{fr} = - \mu mg$, and then substituting above gives $-\dfrac{1}{2}mv_i^2 = -\mu mg d$, and this leads to the final expression in the video for initial velocity. That's a lot of extra trouble to follow the negative signs, which is why I skipped it, but following the negatives is more rigorous, so it's good stuff.

hello :).., i didnt understand why we take the final position as a reference instead of the initial point ..i thought that we always take initial point as a referance

I apologize but I made a mistake with my original question. I multiplied 1/2 K value (31.3 N m) by the amplitude squared (.65), not by the equilibrium value x=.36 m.
Sorry about that!

Hi Professor Ditchko,
For part c, (solving for the total energy), I set the force of gravity equal to the spring force (with x = 0.36 m) and solved for k (31.3 N m). I then used the total energy equation 1/2(K A^2) and came up with 6.6 J. Why is this wrong? Can I not use the given x=0.36 as the equilibrium position here?

Hi moonpen, thanks for the question. A couple of ways to think about this. First is that the total forces directed down need to equal the total forces up in order for these to be balanced. If there was a 'net' force either up or down then the pole would move up or down. This means the 'y-component' of the hinge upward needs to equal the total weight down of the pole and light.

The second way to think about this is that the resultant hinge force is at some angle to the vertical. The resultant isn't straight up nor straight sideways since it has both 'x' and 'y' components. I think what you're asking about is why not multiply the resultant hinge force by the angle between the pole and the vertical? It's important to notice that the angle of the resultant hinge force is *not along the pole*, so it has a different angle. We don't know what that angle is (although it could be calculated since we know the 'x' and 'y' components). Hope this helps.

Hi dsgarrido17, I can see what you mean since the video suddenly has a different sound and brightness at that point. It's the result of correcting a typo in the video where I stitched two different 'takes' together. It's still talking about part (a) at 3:40.

Hi Professor Dychko,
You said that theta and X are similar and therefore interchangeable which I understand; but why can we sub out amplitude and replace it with theta max? How are the two related?

Hi aheumangutman, thanks for the question. Seeing as $x$ and $\theta$ are interchangable, let's rewrite the position formula for a simple harmonic oscillator in a way that might answer your question. Normally this position is written as $x = A \cos (2 \pi ft)$, which means the position at a particular time is some fraction (since cos is always less than 1) of the maximum position. So let's rewrite that as $x = x_{max} \cos (2 \pi ft)$. If $x=\theta$ then it stands to reason that $x_{max} = \theta_{max}$ because we're considering $\theta$ to represent a 'position' now, and if the formula is asking for the maximum position (normally called the amplitude), then that's where we write $\theta_{max}$.

Hello Mr. Giancoli . Could you help me please i have final tomorrow and i cant solve one problem ., i have no one to ask

A 30-g mass moving in positive direction collides head-on with a 10-g mass moving at 60 cm/s in the negative direction. The two masses stick together after the collision, and their velocity is 0.10 m/s. Find the velocity of 30-g mass before collision.

I converted : 30 g = 0.03 kg ; 10 g= 0.01kg ; V2 = - 0.6 m/s ; V prime = 0.1 m/s

I found P = 0.004, i have problem to find V1 of mass 0.03 kg

I thought I would let you know, for some reason you wrote the answer for b) is 1.95 rad, however in the textbook and in your video it is stated the answer is 1.05 rad.

Hi moopen, thanks for letting me know about this typo. The '9' is right beside the '0' so that must be why I accidentally entered 1.95 instead of 1.05.

Hi merkinthedark, thank you for spotting this error. To get the final answer I should have solved for $\dfrac{v_{rms_1}}{v_{rms_2}}$ which would give $\sqrt{\dfrac{m_2}{m_1}}$, and then plugged the numbers as shown in the video. The final answer would be correct for $\dfrac{v_{rms_1}}{v_{rms_2}}$. If we pause to apply the "reality check" to the answer (by asking yourself "self, does this number make sense?") we would expect the isotope with less mass to be faster than the heavier isotope since the speed depends only on temperature (which is the same for each in this question), and inversely proportional to mass (meaning smaller mass gives higher speed) according to $v_{rms}=\sqrt{\dfrac{3kT}{m}}$. This means, with the numbers plugged in the way they are in the video, this should be solving for $\dfrac{v_{rms_1}}{v_{rms_2}}$, not $\dfrac{v_{rms_2}}{v_{rms_1}}$. I've flagged this video for a retake one day and made a note in the quick answer section.

## Giancoli 7th Edition, Chapter 16, Problem 4

By joseotilio25 on Sun, 02/28/2016 - 09:28how I know the values of q1 and q2 because they dont mention in the exercice

Hi joseotillio25, thanks for the question. Since the problem says we're working with

protons, that means the charge on each is theelementary chargeof $1.6 \times 10^{-19} \textrm{ C}$.All the best,

Mr. Dychko

## Giancoli 7th Edition, Chapter 16, Problem 14

By merkinthedark on Wed, 02/24/2016 - 15:15at the mark of 4:23 should it be R12^2+R23^2=R13^2? which would still come out to 2R12^2 but it did confuse me.

Hi merkinthedark,

Thanks for spotting that. Yes, you're quite right that there's a small error in the subscripts, and it should be $r_{12}^2 + r_{23}^2 = r_{13}^2$. I've made a note in the quick answer for other students.

All the best with your studies,

Mr. Dychko

## Giancoli 7th Edition, Chapter 7, Problem 39

By grjoav on Sun, 02/21/2016 - 16:44I have also noticed that this answer is incorrect from the book's answer key section. The given information does not seem to add up to Giancoli's question #39, I believe this may be a completely different question. I am using the seventh edition book and I believe it is not the "Global Edition", although the other questions have been correct so far.

Hi grjoav, thanks for the comment. In the Global Edition, Chap. 7 #39 is about a baseball hitting a brick, whereas in the regular 7th Edition Chap. 7 #39 is about two cars colliding, as covered in the video. What is the difference you're finding between what's in your text versus what's covered in the video?

## Giancoli 7th Edition, Chapter 24, Problem 4

By jiwook.kim on Fri, 02/19/2016 - 23:57its .62mm

## Giancoli 7th Edition, Chapter 24, Problem 4

By jiwook.kim on Fri, 02/19/2016 - 23:57distance is wrong

Hi jiwook.kim, thanks for the comment. At 2:58 in the video I noticed the error in the separation between slits that you pointed out and changed it to $0.62 \textrm{ mm}$, and the correct value was used in the later calculations so the final answer is still correct. Thanks for mentioning this anyways since if there was a mistake then it would be important to let other students know.

All the best,

Mr. Dychko

## Giancoli 7th Edition, Chapter 19, Problem 34

By ctlawson on Thu, 02/11/2016 - 03:33Hello,

I think you went over this in previous videos, but does it matter which "loops" you choose? For instance, I chose the top loop and the bottom loop instead of the top loop and larger outer loop, but my values were slightly off. Should I have gotten the same answer? Thanks in advance for your help.

Hi ctlawson, it doesn't matter which loops you choose. Your goal with 'choosing loops' is to create a collection of equations on which you can then do algebra to figure out the unknowns. You need as many equations as you have unknowns. Each loop is an example of Conservation of Energy, in that the total gain and loss in potential around a closed loop must equal zero. You begin the loop at some point, travel along, and then return to where you started, but if in doing so you arrived with a different amount of energy than when you started, then some energy must have been created or destroyed along the way, which is a violation of Conservation of Energy. Anyhow, I say all this to try and be convincing that the choice of loop doesn't matter. The discrepancy between our answers could be due to intermediate rounding error, or check that you included the $1.0 \Omega$ resistors, or something like that.

All the best,

Mr. Dychko

## Giancoli 7th Edition, Chapter 19, Problem 29

By ctlawson on Thu, 02/11/2016 - 03:03Oh, I see now. Thanks for your help.

## Giancoli 7th Edition, Chapter 19, Problem 29

By ctlawson on Wed, 02/10/2016 - 04:49Hello,

At 6:09, you added 1.3235 and 6.8421 to get 8.1556 and then you added -1.4118 to -2.5263 to get 4.9381. Should you have gotten 3.9381(I3)=8.1656 instead?

Thanks in advance for your help.

Hi ctlawson, thanks for the question. At 6:09 two things were done at once: $-1.4118I_3 - 2.5263I_3 = -3.9381I_3$ as you say, but the second thing done is that this quantity was moved to the other side of the equation. There is the term $1I_3$ over there, so the result is $1I_3 + 3.91381I_3 = 4.91381I_3$ on the left side.

Hope that helps, and all the best with your studies,

Mr. Dychko

## Giancoli 6th Edition, Chapter 17, Problem 16

By thesouthportschool on Tue, 02/09/2016 - 16:24How come the work is positive and not negative?

no one knows

Hi thesouthportschool, the negative sign in $W= -qV$ that you're used to seeing is the work done by the electric field. Since, in this question, we're asked to find the work done by an external force, the external force is applying a force in the opposite direction to that of the electric field. For this reason the work done by the external force has a sign opposite to that of the electric field, which means $W_{ext} = qV$ with no negative sign.

Hope that helps,

Mr. Dychko

## Giancoli 6th Edition, Chapter 8, Problem 38

By gregory.wallace on Mon, 02/08/2016 - 08:11Why do you use r1 and not the sum?

Hi gregory.wallace, since the elbow is chosen as the pivot, we need to measure each 'lever arm' by starting at the elbow. $r_1$ is measured starting at the elbow.

Cheers,

Mr. Dychko

Doesn't the force of the muscle also exert a torque? Wouldn't the torque be the sum of the torque exerted by the weight of the ball and the muscle?

## Giancoli 7th Edition, Chapter 16, Problem 18

By williams.dpw on Tue, 01/26/2016 - 22:55It seems that there is a small typo in your answer for the location for the charge. In the video you say that the location is .37l, not .27l. (~4:10 in video)

Hi williams.dpw, thank you very much for spotting that. I have updated the quick answer.

All the best,

Mr. Dychko

## Giancoli 7th Edition, Chapter 12, Problem 33

By ceward on Tue, 01/26/2016 - 18:22The answer in the back of the book is 0.22m for part A

Hi ceward, thanks for the comment. I'm pretty sure in this case the solution in the back of the book is not correct. I double checked the calculations in the video, and it all checks out. Let me know if you notice any error in the working.

Best wishes,

Mr. Dychko

## Giancoli 7th Edition, Chapter 13, Problem 35

By jimh on Mon, 01/25/2016 - 14:37Unless I am missing something, you have a silly math error here in comparing the value predicted from the gas law, 0.588, to the real value, 0.598. The difference is 0.01, not 0.02 as you wrote it.

Thanks a lot jimh, you're quite right that there is a silly math error. The difference is $0.01 \textrm{ kg/m}^3$ as you said.

All the best,

Mr. Dychko

## Giancoli 7th Edition, Chapter 16, Problem 33

By ctlawson on Wed, 01/20/2016 - 18:15Hi,

Could you explain why you used 30 degree angles instead of 60?

Thank you.

Hi ctlawson, as part of our job in this question we need to break the electric fields into 'x' and 'y' components. It would be perfectly OK to use $60^\circ$. The difference is that you would have to think about which trig. function is the right one to use. For example, when I calculate $E_{Bx}$ using $\cos(30^\circ)$ you could instead use $\sin(60^\circ)$. It's just a matter of personal preference.

Cheers,

Mr. Dychko

Thank you, Mr. Dychko.

## Giancoli 7th Edition, Chapter 19, Problem 28

By brandonsugarman on Mon, 01/18/2016 - 20:35theres a part b i think you neglected to answer ;)

Hi jamesswaggernaut, darn, you're right. The process to solve it for part b) is exactly the same as what's shown for part a). Since the internal resistances are in series with the resistor in the same branch as the battery, the only difference compared with a) is to add one more ohm to $R_1$ and to $R_3$. I've made a note about this in the quick answer, and thanks for spotting it.

Best wishes,

Mr. Dychko

## Giancoli 7th Edition, Chapter 16, Problem 32

By maggiemay480 on Mon, 01/18/2016 - 14:28How do we know that E1 and E2 have the same magnitude? Is that always the case, even though one is closer and one has a stronger charge than the other?

Hi magiemay480, thanks for your question. E1 and E2 having equal magnitude is a special case for this question. The reason for it is that the electric field at point "P" is zero, and the only way for that to be possible is if E1 and E2 are of equal magnitude in opposite directions.

Hope that helps,

Mr. Dychko

Thanks so much, Mr. Dychko! You were a lifesaver in my last course and are continuing to be invaluable in E&M as well! :)

## Giancoli 7th Edition, Chapter 6, Problem 27

By sueqrahn on Sat, 01/09/2016 - 20:15This answer should only have the units of meters, not meters per second

Hi sueqrahn, I've corrected the typo. Thanks for spotting that.

All the best,

Mr. Dychko

## Giancoli 7th Edition, Chapter 6, Problem 23

By mschick16 on Fri, 01/08/2016 - 15:17Why are you allowed to take the absolute value of KE? Shouldn't it be -1/2mvi^2 instead of positive?

Hi mschick16, thanks for the question. I suppose it was unnecessary to take the absolute value of KE. You can safely ignore that part. It's just the way I was thinking about the problem in terms of magnitudes, rather than bothering with direction. Consider this different approach that I think you'll prefer in which I'm following the negative signs properly:

I think you'll agree that $\Delta KE = -\dfrac{1}{2}mv_i^2$, and the video shows that $\Delta KE = F_{net}d$, so this means $-\dfrac{1}{2}mv_i^2 = F_{net}d$. This means $F_{net}$ is negative, which we expect from the diagram since it's the friction force directed opposite to the direction of the initial velocity. $F_{net} = 0 - F_{fr}$ where I'm taking all the forces to the right (of which there are none, hence the zero), minus the forces to the left (just friction). Therefore $F_{net} = -F_{fr} = - \mu mg$, and then substituting above gives $-\dfrac{1}{2}mv_i^2 = -\mu mg d$, and this leads to the final expression in the video for initial velocity. That's a lot of extra trouble to follow the negative signs, which is why I skipped it, but following the negatives is more rigorous, so it's good stuff.

All the best,

Mr. Dychko

## Giancoli 7th Edition, Chapter 6, Problem 41

By j1126990 on Sat, 12/26/2015 - 11:41hello :).., i didnt understand why we take the final position as a reference instead of the initial point ..i thought that we always take initial point as a referance

to keep the numbers positive. positive 15 instead of negative 15

3 years late lol

## Giancoli 7th Edition, Chapter 11, Problem 14

By aheumangutman on Mon, 12/21/2015 - 19:07I apologize but I made a mistake with my original question. I multiplied 1/2 K value (31.3 N m) by the amplitude squared (.65), not by the equilibrium value x=.36 m.

Sorry about that!

## Giancoli 7th Edition, Chapter 11, Problem 14

By aheumangutman on Mon, 12/21/2015 - 18:59Hi Professor Ditchko,

For part c, (solving for the total energy), I set the force of gravity equal to the spring force (with x = 0.36 m) and solved for k (31.3 N m). I then used the total energy equation 1/2(K A^2) and came up with 6.6 J. Why is this wrong? Can I not use the given x=0.36 as the equilibrium position here?

Thanks so much!

## Giancoli 6th Edition, Chapter 10, Problem 11

By cotter.gina on Mon, 12/21/2015 - 15:35Hello, why haven't you drawn the force of gravity going downward? Does the gauge pressure account for the weight of the car as well? Thanks!

## Giancoli 7th Edition, Chapter 9, Problem 19

By moonpen on Sat, 12/19/2015 - 09:05Why wouldn't you use sin37 in Fp and Ft when calculating the Y component of the hinge in part b?

Hi moonpen, thanks for the question. A couple of ways to think about this. First is that the total forces directed down need to equal the total forces up in order for these to be balanced. If there was a 'net' force either up or down then the pole would move up or down. This means the 'y-component' of the hinge upward needs to equal the total weight down of the pole and light.

The second way to think about this is that the resultant hinge force is at some angle to the vertical. The resultant isn't straight up nor straight sideways since it has both 'x' and 'y' components. I think what you're asking about is why not multiply the resultant hinge force by the angle between the pole and the vertical? It's important to notice that the angle of the resultant hinge force is *not along the pole*, so it has a different angle. We don't know what that angle is (although it could be calculated since we know the 'x' and 'y' components). Hope this helps.

All the best,

Mr. Dychko

## Giancoli 6th Edition, Chapter 4, Problem 37

By dsgarrido17 on Thu, 12/17/2015 - 19:05At 3:40 I think it jumps back to A while talking about B. or skips a step

Hi dsgarrido17, I can see what you mean since the video suddenly has a different sound and brightness at that point. It's the result of correcting a typo in the video where I stitched two different 'takes' together. It's still talking about part (a) at 3:40.

Cheers,

Mr. Dychko

## Giancoli 7th Edition, Chapter 11, Problem 34

By aheumangutman on Thu, 12/17/2015 - 11:13Hi Professor Dychko,

You said that theta and X are similar and therefore interchangeable which I understand; but why can we sub out amplitude and replace it with theta max? How are the two related?

Thanks!

Hi aheumangutman, thanks for the question. Seeing as $x$ and $\theta$ are interchangable, let's rewrite the position formula for a simple harmonic oscillator in a way that might answer your question. Normally this position is written as $x = A \cos (2 \pi ft)$, which means the position at a particular time is some fraction (since cos is always less than 1) of the maximum position. So let's rewrite that as $x = x_{max} \cos (2 \pi ft)$. If $x=\theta$ then it stands to reason that $x_{max} = \theta_{max}$ because we're considering $\theta$ to represent a 'position' now, and if the formula is asking for the maximum position (normally called the amplitude), then that's where we write $\theta_{max}$.

Hope that helps,

Mr. Dychko

## Giancoli 6th Edition, Chapter 5, Problem 39

By rbw2306 on Mon, 12/14/2015 - 19:32Why is r4 equal to the square root of .6^2 + .6^2 ?

Forget that comment, pythagorean theorem

## Giancoli 6th Edition, Chapter 4, Problem 53

By SC on Mon, 12/14/2015 - 08:31Isn´t -Fgx = -mgcos22 and not -mgsin22?

Never mind

## Giancoli 6th Edition, Chapter 4, Problem 5

By rinne13 on Sun, 12/13/2015 - 13:39Hello Mr. Giancoli . Could you help me please i have final tomorrow and i cant solve one problem ., i have no one to ask

A 30-g mass moving in positive direction collides head-on with a 10-g mass moving at 60 cm/s in the negative direction. The two masses stick together after the collision, and their velocity is 0.10 m/s. Find the velocity of 30-g mass before collision.

I converted : 30 g = 0.03 kg ; 10 g= 0.01kg ; V2 = - 0.6 m/s ; V prime = 0.1 m/s

I found P = 0.004, i have problem to find V1 of mass 0.03 kg

thank you so much in advance

yana

## Giancoli 7th Edition, Chapter 8, Problem 1

By moonpen on Fri, 12/11/2015 - 18:39I thought I would let you know, for some reason you wrote the answer for b) is 1.95 rad, however in the textbook and in your video it is stated the answer is 1.05 rad.

Hi moopen, thanks for letting me know about this typo. The '9' is right beside the '0' so that must be why I accidentally entered 1.95 instead of 1.05.

All the best,

Mr. Dychko

## Giancoli 7th Edition, Chapter 13, Problem 53

By Mr. Dychko on Wed, 12/09/2015 - 21:50Hi merkinthedark, thank you for spotting this error. To get the final answer I should have solved for $\dfrac{v_{rms_1}}{v_{rms_2}}$ which would give $\sqrt{\dfrac{m_2}{m_1}}$, and then plugged the numbers as shown in the video. The final answer would be correct for $\dfrac{v_{rms_1}}{v_{rms_2}}$. If we pause to apply the "reality check" to the answer (by asking yourself "self, does this number make sense?") we would expect the isotope with less mass to be faster than the heavier isotope since the speed depends only on temperature (which is the same for each in this question), and inversely proportional to mass (meaning smaller mass gives higher speed) according to $v_{rms}=\sqrt{\dfrac{3kT}{m}}$. This means, with the numbers plugged in the way they are in the video, this should be solving for $\dfrac{v_{rms_1}}{v_{rms_2}}$, not $\dfrac{v_{rms_2}}{v_{rms_1}}$. I've flagged this video for a retake one day and made a note in the quick answer section.

All the best,

Mr. Dychko