# Homework Help: Cannon Velocity after firing Cannonball

1. Dec 12, 2011

### HarleyM

1. The problem statement, all variables and given/known data
A stationary 2000 Kg cannon fires a 25 kg cannonball horizontally at 250 m/s. Find the velocity of the cannon after firing the cannonball . You may assume friction is negligible.

2. Relevant equations

M1V1+M2V2=(M1+M2)V'

3. The attempt at a solution

(25)(250)+ (2000)(0)= (2000+25)V'
V'=(25)(250)/(2025)
V'=3.086 m/s

I just need confirmation as Im not 100% positive with my physics... thanks !

2. Dec 12, 2011

### Delphi51

Why 2025?
I would write:
momentum before shot = momentum after shot
0 = mv + MV
0 = 25*250 + 2000*V

3. Dec 12, 2011

### MrB8rPhysics

The total momentum of the situation before the firing is zero. This means that after the firing, the total must remain at zero. Find the momentum of the cannonball once it has been fired. The cannon must have equal momentum in the opposite direction for momentum to be conserved at zero kgm/s

i.e:

Before: (2000*0)+(25*0) = 0

After: (2000*V)-(25*250) = 0, or 2000V = 6250

Taking the cannons velocity to be positive, and the motion of the cannonball in the other direction to be negative. Should be trivial to solve from there (3.125 m/s).

4. Dec 12, 2011

### HarleyM

LOL because my understanding of momentum is limited.. this makes much more sense and gives a velocity backwards which is consistent.

Thanks!

5. Dec 12, 2011

### Delphi51

Most welcome! It pays to write the general principle down, perhaps highly abbreviated, to orient yourself and your readers. I used to be too brief but had to change when I became a school teacher. Students couldn't understand unless I wrote the whole story.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook