physics question

[vorticon]

luckily not related to relativity, so i should be able to get a pretty straight answer for once...


if you taka bar of iron and a bar of aluminium and drop them, they will strike the ground at the same time. BUT if you attach them, the iron bar will always strike first. why?

[spitfiremkv]

the aerodynamic drag resultant is at the joint of the 2 bars. the weight resultant is somewher eon the iron bar (iron more dense than aluminum). when you have two forces that act on a body but not at the same point, a turning moment is created.

[United]

How are they attached?  If side by side, it would be because the iron is heavier than aluminum.

Its the same as taking a barbell bar and some weight and dropping it.
Once with equal distribution, and once with more weight on one side.  The heavier side will fall first.

If you drop the bars separated, gravity is the only force pushing them down.  When you combine them, you add the fact of weight distribution (not the right terminology) and inertia

I dont know the actual word for it, but its something along those lines.

[vorticon]

k got it, basicly its a see-saw dealy, heavier side swings down and stays there. just have to forget about gravity as its non relevant to the question.

[spitfiremkv]

neah it only happens in the air.

and come to think of it, the aluminum bar will fall slower than the iron bar in air!

in vacuum, the joint bar will fall straight.

[Lizking]

Wrong, ladies.  The only way it will happen  that the iron lands below the aluminum, attached or not,  is if the bars are big enough for  air resistance to come into play.

[spitfiremkv]

thanks for reposting what I just said

[Lizking]

Yep, you had it right.

[vorticon]

Originally posted by Lizking
Wrong, ladies.  The only way it will happen  that the iron lands below the aluminum, attached or not,  is if the bars are big enough for  air resistance to come into play.

they have exactly the same shape and dimension. air resistance would be about the same

[Pei]

Originally posted by vorticon
they have exactly the same shape and dimension. air resistance would be about the same

But the centre of mass  would not be in the geometric centre of the object (because the iron bar is more dense), thus the application of air resistance (mostly a feature of the geometry of the object) would create a turning moment causing the iron part of the object to end up downwards (assuming the object fell long enough to have time to turn). Of course this assumes a uniform object: if one bar was a significant different shape the turning moment might be different.

[JB73]

i am imagining 2 "rods" attached down the length, to make a"double rod" say each has i diameter of 1" and length of 2'

dropping so the profile looks liek 2 oo's....

dont forget to account for the air resistance, the alumium bar has less mass and the air comming up will effect it more than the more massive and "stable" iron, hence the aluminium will slow more slowlyin relation to the iron.

i am BAD at explaining it, but i was taught it, and understand the conpect, just can't verbalize it in a drunken hze.

[Pei]

Originally posted by JB73
i am imagining 2 "rods" attached down the length, to make a"double rod" say each has i diameter of 1" and length of 2'

dropping so the profile looks liek 2 oo's....

dont forget to account for the air resistance, the alumium bar has less mass and the air comming up will effect it more than the more massive and "stable" iron, hence the aluminium will slow more slowlyin relation to the iron.

i am BAD at explaining it, but i was taught it, and understand the conpect, just can't verbalize it in a drunken hze.

Physics and alcohol don't mix! Not that that stopped me at Uni........

[Bluedog]

Seeing you use aluminium and iron as an example, would the pull of the Earth's magnetic field play a part in this at all?

[bunch]

the two variables determining terminal velocity in an atmosphere are density & shape coefficient

[Ruin%20]

well if you attach a the to bars by they`re tips  then then as the new bar (which built of the two bars) accellrate on his way down horizontally will be forced by the drag to change position to a more aerodynamic state since the iron end is heavier it will be effected less by the drag force while the other end will be effected more by  it cousing the bar to spin until the iron end  pointing down in a more aerodynamic way.  
on the other hand if you just drop them separated from each other each bar will land on his own tip.