B.V.B
posted by sachin .
A 10G bullet is short from a 5kg gun with a velocity of 400m/s.What is the speed of recoil of the gun?

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Initial momentum = 0
momentum of bullet = mv
momentum of gun = MV
Using conservation of momentum
mv+MV = 0
therefore
V=mv/M 
The momenta of the bullet and the gun are equal (and opposite in direction) after firing. If Vr is the recoil velocity,
5.0*Vr = 0.01 * 400
Solve for Vr, which will be im m/s.
Note that I had to use the same mass units for bullet and gun.