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# Physics - Paper 8 - Uncertainties Watch

I dont understand why its multiplied by pie! Pie is only a constant and it isnt an uncertainty, so whyyy?

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2. (Original post by jtbteddy)

I dont understand why its multiplied by pie! Pie is only a constant and it isnt an uncertainty, so whyyy?

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You haven't posted the beginning of the question but...
You are confusing absolute uncertainty with % uncertainty.

If the absolute uncertainty in a number is, say, 2.0 ± 0.1
and you multiply it by any other number
the absolute uncertainty is also multiplied by that number.

So if I multiply by 10 I get
20 ± 1

The % uncertainty stays the same.
The % uncertainty in 2.0 ± 0.1 is ±5%

It's the same % uncertainty in 20 ± 1
3. (Original post by Stonebridge)
You haven't posted the beginning of the question but...
You are confusing absolute uncertainty with % uncertainty.

If the absolute uncertainty in a number is, say, 2.0 ± 0.1
and you multiply it by any other number
the absolute uncertainty is also multiplied by that number.

So if I multiply by 10 I get
20 ± 1

The % uncertainty stays the same.
The % uncertainty in 2.0 ± 0.1 is ±5%

It's the same % uncertainty in 20 ± 1
Okayy. And if the other value, e.g. If 10 is the length of something with uncertainty +-0.2 mm, id add the uncertainties of both values to get the conbined uncertainties?

And what if i would do pie/2, would i divide pie by the uncertainty of 2 to get the absolute uncertainty? Similarly, what would i do if id add or subtract both values?

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4. (Original post by jtbteddy)
Okayy. And if the other value, e.g. If 10 is the length of something with uncertainty +-0.2 mm, id add the uncertainties of both values to get the conbined uncertainties?

And what if i would do pie/2, would i divide pie by the uncertainty of 2 to get the absolute uncertainty? Similarly, what would i do if id add or subtract both values?

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The rules are
If you have two values with uncertainties and you need to add the values together you also add the uncertainties.
You also add the uncertainties if you have to subtract the values.

If you multiple or divide two values you add the % uncertainties in those values to get the % uncertainty in the result.
5. (Original post by Stonebridge)
The rules are
If you have two values with uncertainties and you need to add the values together you also add the uncertainties.
You also add the uncertainties if you have to subtract the values.

If you multiple or divide two values you add the % uncertainties in those values to get the % uncertainty in the result.
So if i add or subtract, i add the absolute uncertainties and when i multiply or divide, i add the percentage uncertainties?

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6. (Original post by jtbteddy)
So if i add or subtract, i add the absolute uncertainties and when i multiply or divide, i add the percentage uncertainties?

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Exactly.
That's the rule.
This question comes up again and again on here, so a quick search for "uncertainties" in this forum will probably dig up a number of threads with examples of this which might help.
7. (Original post by Stonebridge)
Exactly.
That's the rule.
This question comes up again and again on here, so a quick search for "uncertainties" in this forum will probably dig up a number of threads with examples of this which might help.
Ill do that now- thanks

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