# Initial Velocity Question

1. Nov 3, 2011

### Koi9

So the question I'm asking about is actually from my analytical mechanics class, but I think my question is really more of a gen. physics question.

Basically, in this question we have a cannon ball, diameter(EDIT:but not mass!), fired with a given muzzle velocity, and we are given the cannon mass which is free to recoil. Does the fact that the cannon is free to recoil make it so that I can not use the given muzzle velocity for my initial velocity? EDIT:I would say I should use cons. of momentum (which i did at first)
, but the mass of the cannonball is not given.
Thanks guys,
Matt

Last edited: Nov 3, 2011
2. Nov 3, 2011

### PeterO

Interesting point. Usually the mass of the gun/cannon is so much bigger than the bullet/ball that any difference is too small to worry about.

We often have a cannon of mass XX tonnes, while the ball only 1 or 2 kg, if that?

I would tend to use conservation of momentum just to be on the safe side: they did stress the ability to re-coil.

3. Nov 3, 2011

### Koi9

Thank you for the response, sorry but I completely messed up when stating my givens. All I know about the cannonball is that it is a "10cm diameter steel cannonball," I do not know its mass.

So unless my teacher expects us to look up the density of steel and calculate the mass (which I highly doubt), do you now think it would be safe to assume that the cannons weight and ability to recoil is irrelevant?

4. Nov 3, 2011

### PeterO

What mass did they give for the cannon. Some people give very unrealistic masses in questions. I have seen people claim cars of mass 600kg and trucks of mass 1200 kg before - along with train carriages of mass 2 tonnes.

5. Nov 3, 2011

### Koi9

the mass of the cannon is 40kg.

muzzle velocity of the cannonball is 100m/s

6. Nov 3, 2011

### PeterO

Not much of a cannon!!!!

density of steel is up to 8000kg per cubic metre, so a cannon ball that size has a mass around 30 kg.

You would definitely have to allow for the recoil.

7. Nov 3, 2011

### Redbelly98

Staff Emeritus
Actually, that is a pretty reasonable thing for a teacher to expect. See if your physics textbook has a table or list of densities for various materials (check in the index under "density").

8. Nov 3, 2011

### PeterO

By not much of a cannon, I mean notice that the entire cannon is moulded from 1.3 cannon balls worth of steel!!!!
As I said, people often use very unrealistic masses for objects in Physics problems.