A ball moving horizontally with a velocity $2 \mathrm{~ms}^{-1}$ strikes the bob of a seconds pendulum at rest. If the mass of the bob is equal to the mass of the ball and the collision is perfectly elastic, after collision the bob of the pendulum will raise to a height of
$\left(\mathrm{g}=10 \mathrm{~ms}^{-2}\right)$