# Physics isa Help

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#1
can someone help me with this isa question I do half of it and cant do and understand how the second half is done. The question is

A student doing a similar experiment measures the diameter of the bore of the syringe to be 12.2 ±0.1mm and the unloaded length L to be 55±2mm.
Use these results to calculate the volume of the air in the unloaded syringe and the uncertainty in this volume. (Given that V = πr2h from the physics data sheet).
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6 years ago
#2
Uncertainty is combined by adding the % uncertainties in the two quantities when they are multiplied or divided in a formula.

In your formula for volume the quantities are multiplied so add the % uncertainties in r and h to get the % uncertainty in V.
As r is squared you need to double its % uncertainty.

This is a standard procedure in practical physics.
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6 years ago
#3
Why would you use absolute uncertainty? Is there any specific explanation or any justification to use absolute uncertainty?
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6 years ago
#4
(Original post by thestudent_09)
Why would you use absolute uncertainty? Is there any specific explanation or any justification to use absolute uncertainty?
Yes, when you add or subtract two values, each with an absolute uncertainty, you add the absolute uncertainties to get the total uncertainty.

This is the rule.

When multiplying or dividing values, add the % uncertainties.
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6 years ago
#5
(Original post by Stonebridge)
Yes, when you add or subtract two values, each with an absolute uncertainty, you add the absolute uncertainties to get the total uncertainty.

This is the rule.

When multiplying or dividing values, add the % uncertainties.
What about the uncertainty where you calculate the range of measurements and divide by 2? Is there a specific reason why you would use this uncertainty?
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6 years ago
#6
is the other isa on refraction?
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6 years ago
#7
It is not permitted on this forum to disclose details of practical exams.
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6 years ago
#8
(Original post by thestudent_09)
What about the uncertainty where you calculate the range of measurements and divide by 2? Is there a specific reason why you would use this uncertainty?
Are you asking why uncertainty is used in practical physics?
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6 years ago
#9
(Original post by Stonebridge)
Are you asking why uncertainty is used in practical physics?
yhhh
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#10
(Original post by Stonebridge)
Uncertainty is combined by adding the % uncertainties in the two quantities when they are multiplied or divided in a formula.

In your formula for volume the quantities are multiplied so add the % uncertainties in r and h to get the % uncertainty in V.
As r is squared you need to double its % uncertainty.

This is a standard procedure in practical physics.
Hi im still getting stuck on these types of questions ive been stuck on this question for about an hour http://filestore.aqa.org.uk/subjects...JUN11-TEST.PDF question 3e could you please help me it would be much appreciated.
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6 years ago
#11
Rearrange the formula for k (which you did in the previous part) and find its value.
On the other side of the formula you have T², m, x and L either multiplied or divided.
So the % uncertainty in k is the sum of the % uncertainties in m, x, and L plus 2 times the % uncertainty in T (as it's T²).
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#12
(Original post by Stonebridge)
Rearrange the formula for k (which you did in the previous part) and find its value.
On the other side of the formula you have T², m, x and L either multiplied or divided.
So the % uncertainty in k is the sum of the % uncertainties in m, x, and L plus 2 times the % uncertainty in T (as it's T²).

yes i did that and got the percentage uncertainty to be 5.1% but it isnt asking for the percentage uncertainty but for the uncertainty of the constant which is apparently 2 i dont know how they got to that.
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6 years ago
#13
(Original post by mohamedksmaar)

yes i did that and got the percentage uncertainty to be 5.1% but it isnt asking for the percentage uncertainty but for the uncertainty of the constant which is apparently 2 i dont know how they got to that.
Well what is 5% of that value?

If I had a value of, say, 200 and it was ± 5% then I would find 5% of 200, which is 10, and then say the value is

200 ±10
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#14
(Original post by Stonebridge)
Well what is 5% of that value?

If I had a value of, say, 200 and it was ± 5% then I would find 5% of 200, which is 10, and then say the value is

200 ±10
Yes but I dont already have that value im asked to work it out . The question wants the uncertainty of k but I go up to working out the perentage uncertainty and then dont know how to convert it. Thanks
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6 years ago
#15
(Original post by mohamedksmaar)
Yes but I dont already have that value im asked to work it out . The question wants the uncertainty of k but I go up to working out the perentage uncertainty and then dont know how to convert it. Thanks
You asked how to do 3e
I assumed you had done 3d which involves actually calculating the value of k.
Just rearrange the formula and plug in the values you have been given to find k.
Then do what I said in my last post to find the absolute uncertainty from the % uncertainty.

Either that or I really don't get what it is you are asking. 0
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