# A ball with a mass of 7 kg moving at 6 m/s hits a still ball with a mass of 9 kg. If the first ball stops moving, how fast is the second ball moving?

$4.67 \frac{m}{s}$
$M o m e n t u m = M a s s \cdot V e l o c i t y$
$7 \cdot 6 = 9 \cdot v e l o c i t {y}_{b a l l 2}$
$v e l o c i t {y}_{b a l l 2} = \frac{7 \cdot 6}{9} = 4.67 \frac{m}{s}$