# How Do You Calculate Percentage Uncertainty for Average Speed?

• _Mayday_
In summary, if you measure the time of something to one decimal place, your absolute uncertainty will be 0.05 seconds.
_Mayday_
[SOLVED] Percentage uncertainty

If I measure the time of something to one decimal place, let's say I get 0.6 seconds. Would my absolute uncertainty for this be 0.05 seconds? I have read that it should be 0.01 seconds but that does not make sense to me. Surely my figures form that result could range from 0.55 to 0.65 so that is 0.05 seconds either way?

My actual question here is measuring the percentage uncertainty. If I measure a distance of 0.150 metres, with an absolute uncertaintly of 0.001 metres and a time of 0.6 with an absolut euncertainty of 0.05, what would be the percentage uncertaintly of the result of the average speed.

I am measuring time to a tenth of a second, and distance to the nearest millimetre.Here is my attempt.

$$speed=\frac{distance}{time}$$

% uncertainty for the time is $$\frac{0.05}{0.6}$$

% uncertainty for the distance is $$\frac{0.001}{0.150}$$

Then to calculate the percentage uncertainty of the speed calculated I would plug those figures into the formula?Any help would be great!

Last edited:
I can't imagine it is particularily hard, I'm just having trouble with wether what I am doing is correct, it's more a question of confidence in my own ability. I do not know if what I have done is correct.

I'm going to have to guess, I've done my research on the web and in my book and can't find an answer.

_Mayday_ said:
If I measure the time of something to one decimal place, let's say I get 0.6 seconds. Would my absolute uncertainty for this be 0.05 seconds? I have read that it should be 0.01 seconds but that does not make sense to me. Surely my figures form that result could range from 0.55 to 0.65 so that is 0.05 seconds either way?

You are correct.
_Mayday_ said:
Here is my attempt.

$$speed=\frac{distance}{time}$$

% uncertainty for the time is $$\frac{0.05}{0.6}$$

% uncertainty for the distance is $$\frac{0.001}{0.150}$$

Then to calculate the percentage uncertainty of the speed calculated I would plug those figures into the formula?
Note that you have calculated the fractional uncertainty in your quantities, not the percentage. In general, when performing multiplication or division with quantities that have an associated error you must add them in quadrature. So for your example above let $s,d,t$ be speed, distance & time and $\Delta s, \Delta d, \Delta t$ be their absolute uncertainties respectively. Then;

$$\frac{\Delta S}{S} = \sqrt{\left(\frac{\Delta d}{d}\right)^2 + \left(\frac{\Delta t}{t}\right)^2}$$

I'm sorry that I don't have time for a more detailed explanation right now since it's late. I'll check back in tomorrow and try and add some more.

Hootenanny said:
I'm sorry that I don't have time for a more detailed explanation right now since it's late. I'll check back in tomorrow and try and add some more.

I don't think I can put in words how grateful I am for your help, this is very much last minute stuff as I only got one night to do this work. Thank you.

## What is percentage uncertainty?

Percentage uncertainty is a measure of the possible error or variation in a measurement or calculation. It is expressed as a percentage of the measured or calculated value.

## How is percentage uncertainty calculated?

Percentage uncertainty is calculated by dividing the absolute uncertainty (the smallest possible value that the measurement could be) by the measured or calculated value, and then multiplying by 100.

## Why is percentage uncertainty important in scientific measurements?

Percentage uncertainty is important because it allows us to understand the accuracy and reliability of our measurements. It also helps us to compare different measurements and determine which is more precise.

## What factors can contribute to percentage uncertainty?

There are several factors that can contribute to percentage uncertainty, including the precision of the measuring instrument, human error, and the variability of the environment in which the measurement is taken.

## How can percentage uncertainty be reduced?

Percentage uncertainty can be reduced by using more precise measuring instruments, taking multiple measurements and averaging them, and minimizing sources of human error. It can also be reduced by increasing the number of significant figures in the measurement or calculation.

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