# Homework Help: Calculating recoil

1. May 31, 2015

### TT0

1. The problem statement, all variables and given/known data
A 20kg cannon ball is fired from a 600kg cannon, with an initial velocity of 450 m/s. Calculate the initial velocity of recoil of the cannon.

2. Relevant equations
Conservation of momentum formula

m1u1 + m2u2 = m1v1 + m2v2

where
m = mass
u = initial velocity
v = final velocity

3. The attempt at a solution
m1u1 + m2u2 = m1v1 + m2v2

let v = recoil velocity

20 x 0 + 600 x 0 = 20 x 450 + 600 x v
0 = 9000 + 600v
-9000 = 600v
v = -15 m/s

The negative is from the negative direction so the answer is 15 m/s. Is my logic correct, because I know how momentum is conserved, but is this the right way to think about it? Also how can Newton's law explain this?

Thanks

2. May 31, 2015

### ArmanCham

Yeah your idea is correct to solve this problem. Newton third law says
"When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body."
This is the thing happens here.You exert a force and cannon ball fired.Canon ball goes forward but canon goes back for this affect.Newton third law explain this easily.

3. May 31, 2015

### TT0

Ok thanks a lot