Question

a bullet of mass 100g is fired from a gun of mass 20 kg with velocity of 100 m/s . calculate the velocity of recoil

Answer 1

Recoil velocity is
$v \: = - 1 \div 2$

Answer 2

if it is mcq the recoil velocity is always negative(general point).
now 
we apply conservation of momentum
so we get m1v1+m2v2=m3v3+m4v4
        m1(0.1kg)
        v1(0m/s)
        m2(20kg)
        v2(0)
above values are these because initially everything is at rest
the the final values 
        m3(0.1kg)
        v1(100m/s)
        m4(20kg)
        v4(x)

        0=10+20x
        -0.5=x
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