Dropping Balls!!

[Gareth56]

I read in my physics book the following which is confusing me slightly.

 

“Consider a pair of tennis balls, one a regular hollow ball and the other filled with iron pellets. Although they are the same size, the iron filled ball is considerably heavier than the regular ball. If you hold them above your head and drop them simultaneously, you’ll see that they strike the ground at the same time. But if you drop them from a greater height- say from the top of a building you’ll note the heavier ball strikes the ground first. Why?”

 

It then goes onto explain why and I can follow and understand the argument with no problem BUT what is confusing me is that I’ve heard that if you drop two objects (of differing mass) from the same height then they’ll hit the ground at the same time due to the heavier one having more inertia as performed by Galileo Galilei's dropping objects from the leaning tower of Pisa in order to prove that all objects fall at the same rate, whatever their mass.

 

 

So I’m not too sure why the text in the book tells me that two objects of differing mass will hit the ground at different times. Am I misunderstanding the text in the book or what?

[insane_alien]

you aren't considering all the variables.

 

the variable you are missing in this case is friction with the air. it isn't very prominent at low velocities so dropping them from head height will not result in significantly different times of impact but from a large building where the lighter ball is likely to hit its terminal velocity(or get close to it) then the difference will be very noticable.

 

if it was in a vacuum then they would hit the ground no matter what the height.

 

an experiment which exemplifies this is a hammer and a feather. this was done on earth and the moon. in the atmosphere the feather falls slowly to the ground(high air friction) but on the moon if fell just as fast as the hammer.

[Gareth56]

That's what the explanation went on to say in the book but what was confusing me was the Galileo experiment and having seen (in the flesh as it were) the Leaning Tower of Pisa it does seem (to me anyway) that it would have been high enough to show that heavier objects do drop faster in the presence of air.

[insane_alien]

yes, but gallileo used iron cannon balls which are far more dense than a tennis ball so the terminal velocities of those are far far higher than the tennis balls. this means it would have had to drop even furth to show an effect. a few thousand feet would probably be required. i've seen the tower of pisa too and it ain't a few thousand feet.

[Gareth56]

~ 185 feet I think

 

Thanks

[swansont]

Also important was the model that dropping speed varied with mass, in some proportion, was falsified. Something with twice the weight does not drop twice as fast. So even if the heavy ball were seen to be dropping slightly faster, a whole range of models are shown to be incorrect.

[Externet]

Inflate a balloon with air.

Fill an identical balloon to the same exact volume with water

 

Drop them simultaneously from your hands from exactly the same height. No need to climb on a building.

 

They are both same size, same shape, same coheficient of friction. They have different masses.

 

Check if they hit the floor at the same time

[John Cuthber]

"If you hold them above your head and drop them simultaneously"

They will land on your head.

 

(for a give definition of "above".)

Anyway, Galileo already knew that the rate of fall couldn't be proportional to the mass. He conmsidered two balls tied together with a thin string then dropped.

The combined object "ought" to fall twice as fast as one of the balls on its own, but the thin string couldn't make any difference.

The idea that they "ought" to fall twice as fast didn't make sense anyway.