# Initial Velocity Question

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

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PeterO
Homework Helper
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, mass given, 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? As in do i have to do some cons. of momentum to get the actual v initial?

Thanks guys,
Matt

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.

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.

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?

PeterO
Homework Helper
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?
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.

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.

the mass of the cannon is 40kg.

muzzle velocity of the cannonball is 100m/s

PeterO
Homework Helper
the mass of the cannon is 40kg.

muzzle velocity of the cannonball is 100m/s

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.

Redbelly98
Staff Emeritus
Homework Helper
...So unless my teacher expects us to look up the density of steel and calculate the mass (which I highly doubt), ...
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").

PeterO
Homework Helper
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.

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.