Assume we have a cannon which has a mass of 572 stones. Stones is an English unit. Which fires a projective which has a mass of 40 pounds horizontally. The projective has a velocity of 9,000,000 football fields/fortnight. (A football field is 120 yards. A fortnight is 14 days.) What is the recall velocity of the cannon in nm/decade?
The equation to use is momentum conservation:
M Vr = m Vp
Vr is the recoil velocity of the cannon (mass M) and Vp is the velocity of the projectile (mass m). You will have to convert your various weird units to a common set of dimensions, such as meter/kg/second. That will give you an answer in m/s. You will have to multiply that by 10^9 nm/m * 3.156*10^8 s/decade to get the recoil velocity in nm/decade.