A body of 2 kg mass slides down with an acceleration of $4 \mathrm{~ms}^{-2}$ on an inclined plane having slope of $30^{\circ}$. The external force required to take the same body up the plane with same acceleration will be (acceleration due to gravity $=10 \mathrm{~ms}^{-2}$ )

A body of mass 30 kg moving with a velocity $20 \mathrm{~ms}^{-1}$ undergoes one-dimensional elastic collision with another ball of same mass moving in the opposite direction with a velocity of $30 \mathrm{~ms}^{-1}$. After collision the velocity of first and second bodies respectively are