At GCSE sort of level the beer stays in the can, the skydiver and the can and the beer all falls at the same rate so it stays in there.
In reality there is the atmosphere to consider, as the can falls through the air it forms a higher pressure at it's base and a low pressure at its top which will have the effect of sucking the beer out of the can
Bearing in mind that at 20,000ft atmospheric pressure is around 1/4 of that at the earth's surface so, assuming fizzy beer, it's going to fizz out of the can anyway.
even at O level, I thik para 1 of J t k's post is valid
the law of physics is Newton's 3 rd law -
man-parachute-beer - no internal forces - or all internal forces cancel so they act a fixed body
and para 2 - if Phigh is 0.25 x P sea level then V is 4V - of a bubble in the beer - and that answers jomiflls point of what replaces it ?
Answer: the expanded gas in the bubble
Oh, that is boyle's law
Not a bad question in terms of getting people to think about the basic laws of phys.
If you drop a few thousand feet whilst in the aircraft, the drink stays in the cup. That is assuming that you don`t go up before you come down, of course
If it was just water in the can, as soon as you reached terminal velocity (after a few seconds the water watger would have the same force holding it in the can as if you were standing on the ground.