# Gravity on a lonely planet

Astronauts on an atmosphere-less planet wanted to find the acceleration due to gravity. And to find it, they launched a ball from the Surface of the planet, with a velocity of $$15\text{ m/s}$$. And hence the astronauts predicted that the ball would reach as height of:

$s=15t-\dfrac 12 g_pt^2$

$$t$$ seconds after the launch. If the ball reached the maximum height after $$20$$ seconds. What was the value of $$g_p$$?

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