HELP IN UNCERTAINTY QUESTION PLSSSSSS! AS level physics. Simple if you know it!

[Peter Check]

A student measures the wavelength of waves in a ripples tank using a metre rule which is graduatd in millimetres. Estimate the uncertainty in the measurement.
Can you explain your answer plsss

 

[Mariam Pasha]

Uncertainty is the half of the smallest unit on the apparatus being used...
In this question, the smallest unit is the millimeter, so half a millimeter is your uncertainty - but state it in the form of a meter
so it's going to be:-
(your answer) + or - 0.5 x 10^-3 m

 

[Peter Check]

are you sure its 1/2 the smallest division? because we do that when we are measuring sumthing twice(like in a burette), in this case shouln't we use only the smallest division which we can detect????

 

[Peter Check]

Hey can you look at pg 46 of the file I have uploaded. The part where it says (The percentage uncertainty in the length is 0.34%) They are using 1mm. How come??? Like you said shouln't it be half the smallest division??

 

[Mariam Pasha]

It should be, but if you're taking 2 readings the uncertainty multiplies.
the first reading is when you're placing your zero at the edge of what you're measuring (+ or - 0.5mm) and the next is the reading you're taking (+ or - 0.5mm)
add them together and you get ( + or - 1mm)

 

[Mariam Pasha]

your guide says "If single readings have been taken then the uncertainty should be the smallest interval or division
on the measuring instrument. Consider the example below." clearly emphasizing on the "single" - in a ruler you take two readings

 

[Peter Check]

Mariam Pasha- Dude can you please be online for sometime( like 10 mins) so I can get a clear discussion/understanding of this?

 

[Peter Check]

I thiink this question's answer will clear my doubt. What is the smallest division in a metre rule? .5mm or 1mm?