IB DP Physics Unit 1.2 Uncertainties and errors SL Paper 1


Two sets of data, shown below with circles and squares, are obtained in two experiments.

The size of the error bars is the same for all points.


What is correct about the absolute uncertainty and the fractional uncertainty of the y intercept of the two lines of best fit?

Absolute uncertainty

Fractional uncertainty

A larger for squares


B larger for squares

larger for squares

C same


D same

larger for squares


Ans: D

The uncertainty in a measurement can be shown on a graph as an error bar. This bar is drawn above and below the point (or from side to side) and shows the uncertainty in that measurement

The percentage uncertainty in the gradient is given by = \(\frac{m_2-m_1}{m}=\frac{\Delta m}{m}\times 100\).

In above diagram , \(\Delta m\) is same , hence absolute uncertainty is same but clearly m is low for squares hence percentage uncertainty is larger.


A student measures the length l and width w of a rectangular table top.


What is the absolute uncertainty of the perimeter of the table top?

A 0.3 cm

B 0.6 cm

C 1.2 cm

D 2.4 cm


Ans: B

If two values are added together then the uncertainties also add . 

\(\Lambda y=\Delta a +\Delta b \) 

Hence P = w+w+l+l  or  \(\Delta p= 2 \times \Delta w +2\times\Delta l\) = 2*0.2+2*0.1= 0.6 cm