physics
posted by please help .
A rifle with a mass of 2.0kg is used to fire a bullet with a mass of 50.g. The bullet leaves the gun with a speed of 200. m/s. What is the recoil velocity of the gun?

Use the law of conservation of momentum. The bullet and the gun will have equal and opposite momenta right after the bullet leaves the rifle.
2.0 * Vrecoil = 0.05 kg * 200 m/s
Solve for Vrecoil 
commanly the formula is: *m1u1+m2u2+m1v1+m2v2*
substitute the values according to this formulae.
This data can be likely:
mass of rifle=m1=2.0kg
mass of bullet=m2=50g=kg= 50/1000=0.05kg
velocity of gun=v1=0m/s
velocity of recoil of gun=v2=?