Browse Questions

# A person is driving a car towards east at a speed of 80 km/hr. A train appears to move towards north with a velocity of $80\sqrt{3}$ km/hr to the person driving the car. Find the speed of the train as measured with respect to earth.

$\begin{array}{1 1}160km/hr\\180km/hr\\260km/hr\\280km/hr\end{array}$

Let us first identify the car and train as A and B. Here, we are provided with the speed of car (A) with respect to Earth i.e. and speed of train (B) with respect to A i.e
$v_A=80km/hr$
$v_{BA}=80\sqrt 3$km/hr
We are required to find the speed of train (“B”) with respect to Earth i.e. , . From equation of relative motion, we have :
$\Rightarrow v_B=\sqrt{\big(v_{BA}^2+v_A^2\big)}$
$\Rightarrow \sqrt{\{(80\sqrt 3)^2+80^2}\}$=160km/hr
edited Aug 13, 2014