From the following displacement-time graph find out the velocity of a moving body.

 

At first instance you will apply $\mathrm{\upsilon } = \mathrm{tan\theta } \mathrm{and} \mathrm{say},$

$\mathrm{\upsilon } = \tan {30}^0 = \frac{1}{\sqrt{3} } \mathrm{m}/\mathrm{s}$

But it is wrong because formula $\mathrm{\upsilon } = \mathrm{tan\theta } \mathrm{is} \mathrm{valid} \mathrm{when} \mathrm{angle} \mathrm{is} \mathrm{measured} \mathrm{with} \mathrm{time} \mathrm{axis}.$

Here angle is taken from displacement axis. So angle from time axis = ${90}^0 -{30}^0 = {60}^0$

Now $\mathrm{\upsilon } = \tan {60}^0 = \sqrt{3}$