At 8:08, shouldn't you have rounded the calculated speed to 10. metres per second? The time 3.0s only had 2 significant digits, so wouldn't you need to round the division calculation to two sig digs? Or is it better to not round at all during calculation, leave the mult./div. sig dig rule and keep the same number of decimal places as the uncertainty for the final value of the calculation?
Ignore sig. figs. when you have uncertainties since the uncertainties are more restrictive. Round the absolute uncertainty to 1 or 2 sig. figs, and then round the value to the same place value as the smallest digit in the uncertainty.
Wouldn't option A. also be a correct answer to the last question? Because regardless of whether there is addition or subtraction of quantities the absolute uncertainties are added in both the cases?
Hey Chris, at 13:00 you said that sin(20) = 0.3420 but then rounded it up to 0.39 after. Just thought I should point that out, maybe a reading error? Great video though, thanks :)!
Just to clarify, if the formula includes a constant, do we just disregard it or do we have to multiply that absolute/percentage uncertainty by the value?
(I'm going to specifically suggest an alternative to the sine example because I'm having trouble putting the abstract concept in words) Wouldn't sin20-sin18 be a better value for the uncertainty than (sin22-sin18)/2? If the value has an uncertainty of (max-min)/2, then it shouldn't be possible for it to be sin18, because sin18 < sin20-(sin22-sin18)/2.
Keep in mind that the uncertainty is always a rough estimate. We are just looking for a range of values that we are confident most repeated measurements would lie within. In other words, we don't need to be picky about how we arrive at our uncertainty so long as it is reasonable.
Paper 3 begins with a large data analysis question, and a question or two that relate to common experiments. You will then do your option questions. My students do the SL engineering option which consists of a thermodynamics question and a rotation question.
Sir when you say that for the factional uncertainty you write 6.6 s +/- 0.03 however then the uncertainty does not have the same value as the 6.6 so would this be incorrect or do we only match the decimal places for absolute value uncertainties
Good question. Thank you. It is only for absolute uncertainties (not percent or fractional uncertainties) for which the number of decimal places will be in agreement between the value and the uncertainty. (i.e. suppose the fractional uncertainty were 0.5, and the value were 0.023, it would be nonsense to round to the first decimal place to obtain 0.0 +- 0.5. We don't need agreement because the units are not the same. The value has its units, and the fractional uncertainty has no units.
Thanks Hanbo. That was definitely sloppy on my part. I shouldn't make videos when I am tired. I will add an annotation. Next time let me know the time at which the mistake occurred.
+Joaquin Angulo When you divide, you add the percent uncertainties, not the absolute, so you end up with about a 4% uncertainty of approximately 1. The resultant absolute uncertainty woud then be about 0.04.
Saving us from failing 🙌🏽 thank you!!
You are so welcome!
At 8:08, shouldn't you have rounded the calculated speed to 10. metres per second? The time 3.0s only had 2 significant digits, so wouldn't you need to round the division calculation to two sig digs? Or is it better to not round at all during calculation, leave the mult./div. sig dig rule and keep the same number of decimal places as the uncertainty for the final value of the calculation?
Ignore sig. figs. when you have uncertainties since the uncertainties are more restrictive. Round the absolute uncertainty to 1 or 2 sig. figs, and then round the value to the same place value as the smallest digit in the uncertainty.
In the last question how would I know to subtract 51-49? And aren’t those absolute uncertainties not percentage uncertainties
We are given that a=51 and b=49, so a-b=2. It is absolute. The percentage uncertainty is 100%.
15:06
i know it's not the question but
option c and d got the same percentage uncertainty right?
Correct!
Wouldn't option A. also be a correct answer to the last question? Because regardless of whether there is addition or subtraction of quantities the absolute uncertainties are added in both the cases?
The absolute uncertainty would be the same but the percent uncertainty would be much smaller for a-b.
Thanks a lot for explaining.
For the last question at 15:07, since the question is asking in percentage unc, can you write the answer as 2±100%?
yup.
thank you so much now i am able to do my homework :)
Hey Chris, at 13:00 you said that sin(20) = 0.3420 but then rounded it up to 0.39 after. Just thought I should point that out, maybe a reading error? Great video though, thanks :)!
Yes, having a hard time reading my own writing. Thank you.
but in 13.13 we calculated sine of 20 which was 0.3420 how did we round it to 0.39 is it a mistake or is there something im missing??
Thanks Bryan. It was a mistake, I can't read my own writing. I added an annotation.
Just to clarify, if the formula includes a constant, do we just disregard it or do we have to multiply that absolute/percentage uncertainty by the value?
The percentage uncertainty would remain the same, however, we would need to multiply the absolute uncertainty by the constant.
Hi Chris, at 8:20 how did you convert 69% (percent form) into 0.6 (absolute form)?
Thanks :)
its not a 9, Chris just writes the % inn a weird manner. It got me too at the beginning. :)
(I'm going to specifically suggest an alternative to the sine example because I'm having trouble putting the abstract concept in words)
Wouldn't sin20-sin18 be a better value for the uncertainty than (sin22-sin18)/2? If the value has an uncertainty of (max-min)/2, then it shouldn't be possible for it to be sin18, because sin18 < sin20-(sin22-sin18)/2.
Keep in mind that the uncertainty is always a rough estimate. We are just looking for a range of values that we are confident most repeated measurements would lie within. In other words, we don't need to be picky about how we arrive at our uncertainty so long as it is reasonable.
at 9:20 the plus or minus is only written as plus for the second op. Otherwise, Thanks
Do you know which topic to focus on paper 3? Other than option topics?
topic 1, one option (your choice), and a list of experiments you were meant to do in class are the only things tested on paper 3.
Paper 3 begins with a large data analysis question, and a question or two that relate to common experiments. You will then do your option questions. My students do the SL engineering option which consists of a thermodynamics question and a rotation question.
thanks a lot; really helpful
Glad it helped!
Hi Chris, do we always keep 1 s.f for percentage uncertainty?
If you want a quick fast rule, keep 2 digits in the percent uncertainty, but really it depends on the absolute uncertainty.
Sir when you say that for the factional uncertainty you write
6.6 s +/- 0.03
however then the uncertainty does not have the same value as the 6.6 so would this be incorrect or do we only match the decimal places for absolute value uncertainties
Good question. Thank you. It is only for absolute uncertainties (not percent or fractional uncertainties) for which the number of decimal places will be in agreement between the value and the uncertainty. (i.e. suppose the fractional uncertainty were 0.5, and the value were 0.023, it would be nonsense to round to the first decimal place to obtain 0.0 +- 0.5. We don't need agreement because the units are not the same. The value has its units, and the fractional uncertainty has no units.
Will uncertainties be tested on the IBSL physics exam?
Typically one multiple choice question in paper 1, and one on the data analysis question in paper 3.
Circumference is not centimeters squared- that is only for the area of a two dimensional object.
Thanks Hanbo. That was definitely sloppy on my part. I shouldn't make videos when I am tired. I will add an annotation. Next time let me know the time at which the mistake occurred.
It is alright, I was just just a bit confused. Other than that, this video was really great and helped me a lot on my IB lab. Thank you!
Thanks a lot. However, in the last problem why isn´t it d) "a/b" that gives you approximately 1 plus or minus 2
+Joaquin Angulo
When you divide, you add the percent uncertainties, not the absolute, so you end up with about a 4% uncertainty of approximately 1. The resultant absolute uncertainty woud then be about 0.04.
+Chris Doner (C. Doner's IB Physics) Thanks!!
shouldn't the uncertainty be multiplied by the constant too?
When you multiply by a constant, the absolute uncertainty is multiplied by the constant, however, the percent uncertainty stays the same.
But if the uncertainty is multiplied by the constant, then wouldn’t the value of the percent uncertainty be different ?
Say the uncertainty is 1 in 10 or 10%. If we multiply by a constant, say 2, we would get 2 in 20, which is stil 10%.
oh ok I get it now. thanks sir!
When is the fractional uncertainty to big
At what time are you referring to in the video?
this video was very helpful thank you
sebastiaan kimman hi
help