The final velocity is 2.7 m/s
Explanation:
We can solve this problem by using the principle of conservation of momentum: in fact, in absence of external forces, the total momentum of the system must be conserved before and after the collision.
Therefore we can write:
 
  
where:  
 is the mass of the putty
 is the mass of the putty
 is the initial velocity of the putty (we take its direction as positive direction)
 is the initial velocity of the putty (we take its direction as positive direction)
 is the mass of the ball
 is the mass of the ball
 is the initial velocity of the ball (at rest)
 is the initial velocity of the ball (at rest)
 is the final combined velocity of the two putty+ball
 is the final combined velocity of the two putty+ball
Re-arranging the equation and substituting the values, we find the  final combined velocity:
 
And the positive sign indicates their final direction is the same as the initial direction of the putty.
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