It depends upon the player too.

http://en.wikipedia.org/wiki/Bend_It_Like_Beckham

I fail to see any enhanced enjoyment from wobbly balls.

Why?

Completely filling a ball with a fluid will change the total mass of the ball but I would not expect any significant trajectory changes as it travels through air. There maybe an unpredictable effect bouncing a ball partially filled with a fluid. Making a ball where the center of mass does not coincide with the volumetric center (uneven density) will make for the wobbling effect you wish for.

I am certain though that every sport has limitations if not an out right banning of any modification of a game ball. People have been shot for less. Be very careful who and when you use such a doctored ball.

The rotational inertia would change quite a bit, and in some sports that would matter a lot.

I think it would affect a sport like baseball quite a bit. The internal drag between the fluid and the outer sphere would reduce the curve in curve balls and sliders, and might flatten out the trajectory of a knuckleball, too. Then after the ball has been hit, the torque imparted back to the ball would affect it's recoil. The solid 'crack' for a home run would probably be gone, and perhaps the home run would also be gone. Depending on how rigid the outer shell is, there would likely be less spin, which could affect how a grounder bounces, and so on. It might be like playing with a wiffle ball with a water balloon stuffed inside it.

Take a fresh egg and a hard boiled egg and roll them on the table.

Dave

Almost there...the raw egg can't possibly bounce...just like a tennis ball filled with fluid.

All balls will be googlies.

Responses 5 & 6 to the thread are meaningful. I definitely would understand that speed of games like football, basketball or cricket would be slowed down. There will be fewer goals. In cricket - the ball will spin unpredictably when bowled. So also in foot ball when a player kicks the ball - the trajectory taken will not be straight as predicted - and now we introduce a game of chance in between. the foot ball can change teams depending where it falls and who gets it by watching the trajectory change. That is part of the fun.

When the fluid is 100% filled - it will be as good as a solid ball. Hence the fluid content must be less in volume - to add friction within. When the foot ball of this kind is kicked - the solid outer shell and the liquid therein do takes the force. it is the partly filled fluid - which will decide the trajectory depending its distribution internally.

Thank you for acknowledging that there will be a difference in trajectory- well whether it will be acceptable to sports fraternity is different. May be children can play such a foot ball and have fun- to begin with.