When to use the formula of projectile motion and when to use the formulas of straight line motion, if you are at a height and throwing a stone at ground?

Dear Student,

When you are dropping or throwing the stone towards a point on the ground exactly vertiaclly downwards, then you use the straight line motion equations as the stone would be moving in a vertical direction only. Though if you give some horizontal velocity as well to the stone, then along with a horizontal motion, there would be a vertical motion as well, which results into a projectile motion.

Regards

THANK YOU SIR....!!! 

Dear student,

You can use projectile formula if there is a horizontal velocity imparted to the stone, but if the particle is dropped, then it would be the straight line motion formulae which needs to be applied.

Regards

Sumit