Suppose two points are situated at the origin of xy coordinate system. Two points start moving at the same time. Point $T_1$ starts moving towards north with velocity $v_1 = 2.7m/s$ while point $T_2$ starts moving towards east with velocity $v_2 = 1.6 m/s$ and acceleration $a_2 = 0.9 m/s^2$.
What is their relative velocity at time $t = 10s$?
1. solution:
We can describe point $T_1$ with vector $\vec{v_1} = v_1\hat{j}$ and point $T_2$ with vector $\vec{v_2} = (v_2+a_2t)\hat{i}$
If we now subtract $\vec{v_2}$ from $\vec{v_1}$ we get $\vec{v_r} = v_1\hat{j}-(v_2+a_2t)\hat{i}$
It's length is $\sqrt{v_1^2+(v_2+a_2t)^2}$
2. solution:
We can calculate the distance between two points using pythagorean theorem: $d=\sqrt{(v_1t)^2+(v_2t+\frac{a_2t^2}{2})^2}$
$\textbf{It's derivative is relative velocity.}$
If we now graph those two functions, we don't get the same graph:
Why is there a diference between those two methods?