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Q. A cart is moving horizontally along a straight line with constant speed $30\, m / s$. A projectile is to be fired from the moving cart in such a way that it will return to the cart after the cart has moved $80\, m$. At what speed (relative to the cart) must the projectile be fired $\left(\right.$ Take $\left.g=10\, m / s ^{2}\right):$

Motion in a Straight Line

Solution:

Time of motion,
$t =\frac{ x }{ u _{ x }}=\frac{80}{30}=\frac{8}{3} s$
Thus, $0=u \sin \theta t -\frac{1}{2} gt ^{2}$
$\Rightarrow u \sin \theta=\frac{40}{3}$
$\therefore u _{ y }=40/ 3\, m / s$