A projectile is fired from the surface of the earth with a velocity of $5 \mathrm{m} {\mathrm{s}}^{-1}$ and angle $\theta$ with the horizontal. Another projectile fired from another planet with a velocity of $3 {\mathrm{m} \mathrm{s}}^{-1}$ at the same angle follows a trajectory which is identical with the trajectory of the projectile fired from the earth. The value of the acceleration due to gravity on the planet is: (given $=9.8\mathrm{ }\mathrm{m} {\mathrm{s}}^{-2}$)