Hello, I am not sure if this thread is still active but I am doing the same kind of physics problem for homework, and I think I have come to a conclusion from all the helpful stuff on here, but just wondered if someone can check it over for me? This is not in my GCSE syllabus - my physics teacher just likes to set us O Level work, which is full of stuff no longer in our syllabus!!! So the problem goes: "A gun of mass 4.0kg fires a bullet of mass 10g at a speed of 60 m/s. What is the initital speed of recoil of the gun?" First I thought I would have to convert both masses so they are in the same units, so I converted 4kg to 4000g. From this thread I took "(Mass of gun)*(Speed of Gun) = (Mass of Bullet)*(Speed of Bullet)" Please correct me if I am wrong here. So I substituted in the numbers. 4000*? = 10*60 4000*? = 600 600/4000 = 0.15 Speed of recoil = 0.15 m/s I have no idea if this is correct, please please give me a pointer in the right direction? Thanks, Molly.