I know that Io is volcanic because its elliptical orbit makes it stretch and contract causing friction. But if I look at energy transfer its kinetic to heat energy, but the kinetic energy comes from the elliptical orbit which only exists because of the gravitational pull of Europa and Callipso, yet gravity is one of the four forces of nature and not an energy. How? 

I have come to understand that the effects of gravity extend out almost endlessly but that the attractive force reduces in strength with distance.

Would two small objects, say, two bowling balls located in a completely empty universe, spaced, say, a light-year apart, eventually come together due to gravitational attraction? Is there a minimum mass these objects would need to reach each other gravitationally, or would it just be a question of time? There would be no other forces acting on these objects.

does the strength of gravity fluctuate due to the position of the sun and moon and there gravitational pull on you or is there an equal force on the earth which cancels this effect out?

Recently i have wondered about how the moon and sun affect gravity here on earth. I was wondering wether the force of gravity would change during night or day due to the position of the sun and the moon and their gravitational pulls. But just as I sat down to work this out (being the cool 16 year old I am) I also remembered that the sun and moon would also pull on the earth. so the question is would the pull the on the earth be the same as that on you and therefore cancel out any effects or do you infact become magically (or marginally) lighter at night or day due to this affect?

I'm sure the answer to this will be yes, but is there any experiment or astronomical observation that proves it?  For example, could I weigh a wire,  then weigh it again when a current is passing through it and see a difference?

Is it possible to reliably define the times (for a specific location on earth) when the tidal gravity from astronomical objects (moon, sun etc.) are at their weakest & strongest?

I have considered using sea & ocean tide times, but these factor in unwanted variables, such as ocean currents and coastal water flows, which would invalidate the results somewhat.

I am attempting to define whether any noticeable effects occur from the moon on athletic performance, but need specific times with which to test and compare (hence the strongest & weakest times are my targets).

Is there a reliable method to calculate the times of strongest and weakest gravitational pull of the moon upon objects at a specific location on earth, in effect a high and low tide? We considered using tide times, but they factor in variables such as ocean currents and coastal flows, which would adversly affect our results. Is there a formula that can give us a good answer?

Is there a formula to give the times when the lunar gravitational effects are strongest and weakest for a location on earth? We considered the tide times, but they factor in variables such as coastal flows and ocean currents, which would adversly alter the calculations