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A gun weighing 10 kg fires a bullet of 30g with a velocity of 330 m/s.With what velocity does the gun recoil? What is the combined momentum of the gun and bullet before and after firing?

Here we shall use the conservation of momentum to calculate the recoil velocity. 
Momentum of the bullet + gun before firing = 0 
Momentum of the bullet + gun after firing = (30 × 10^-3) (330) + (10)(v) 
‘v’ is the recoil velocity of the gun. 
So, by momentum conservation, 
(30 × 10 -3 )(330) + (10)(u g ) = 0 
=> u g = - 0.99 m/s 
The negative sign indicates that the gun’s velocity is directed opposite to the velocity of the bullet. 
Now, 

The combined momentum of the bullet and gun before firing = 0

The combined momentum of the bullet and gun after firing =( 30× 10^-3) 330 + 10 × - 0.99

= 9.9-9.9 = 0