Physics question - surely it's just the extra weight?

[Alfanatic]

Einion Yrth said:

Ari said:

Or am I just being thick? (Again).

Thick? Maybe, maybe not. You are plainly, however, ignorant of the fact that in any given gravitational field acceleration due to gravity will be identical regardless of the mass involved.

That's not quite true according to Newton's gravitational law though, is it? Gravitational force depends on the mass of both objects and their distance apart, so the heavier object will feel a greater force. I think it just ends up being identical (practically) when one object is really big (the earth) relative to the other (a dumbbell vs a dumbbell and some chain) so the difference in acceleration is going to be an awful lot of decimal points down the line. If you tried it with three objects of comparable mass, say three planets instead of one planet and two relative specks of material, I reckon the differences in acceleration would be more apparent.

Still, I'm not suggesting that this is behind the difference shown in the video.

[FunkyNige]

Alfanatic said:

That's not quite true according to Newton's gravitational law though, is it? Gravitational force depends on the mass of both objects and their distance apart, so the heavier object will feel a greater force. I think it just ends up being identical (practically) when one object is really big (the earth) relative to the other (a dumbbell vs a dumbbell and some chain) so the difference in acceleration is going to be an awful lot of decimal points down the line. If you tried it with three objects of comparable mass, say three planets instead of one planet and two relative specks of material, I reckon the differences in acceleration would be more apparent.

Still, I'm not suggesting that this is behind the difference shown in the video.

Yes, but if you replace the Gravitational force with mass x acceleration, the mass of falling object cancels out on both sides of the equation.

This site shows it much better than I can without subscripts!

http://faculty.wwu.edu/vawter/PhysicsNet/Topics/Gr...

[Alfanatic]

FunkyNige said:

Yes, but if you replace the Gravitational force with mass x acceleration, the mass of falling object cancels out on both sides of the equation.

This site shows it much better than I can without subscripts!

http://faculty.wwu.edu/vawter/PhysicsNet/Topics/Gr...

Aaah ok thanks for the link. I didn't get it on the first pass but I'll keep trying

[FunkyNige]

Alfanatic said:

Aaah ok thanks for the link. I didn't get it on the first pass but I'll keep trying

Write it out yourself, it makes it much easier to follow what's going on
That's what I always had to do, anyway!

[Alfanatic]

I will do. My problem is that I am expecting to see both object influencing eachother, but in the equation I see the force the big one applies to the small one but not the force the small one applies to the big one. It's been a long time since I studied Newton's laws though... I probably last knew why it makes sense in 1986

[Ari]

Ledaig said:

Irrespective of mass, everything will accelerate at the same rate 9.81m/s^2.

Something to think about is that during the fall the chain will be building up a store of energy as it's velocity increases, as each link in the chain reaches the end of it's permitted travel this energy will be transformed into an alternative form - in this case applied as an additional accelerative force to the remaining length of chain in free fall as well as the weight.

So if the chain is accelerating at the same rate as the dumbell, each link won't be reaching the end of its travel, it'll all be static relative to each other surely?

[Ari]

deadtom said:

I think its kinda related to the feather vs hammer scenario; in this situation both objects have more or less the same atmospheric drag, however the chained weight has more effective mass, meaning the force is greater (F = MA, or in this case Ma where a acceleration due to gravity at earth's surface), so it overcomes the atmospheric drag more easily.

That's kind of how I see it, attaching a length of heavy chain pulls it down quicker.

But the video suggests it's something to do with the chain on the other side of the loop somehow exerting an upward force.

I just can't see it...

[tank slapper]

Ari said:

That's kind of how I see it, attaching a length of heavy chain pulls it down quicker.

But the video suggests it's something to do with the chain on the other side of the loop somehow exerting an upward force.

I just can't see it...

If the chain were released at both ends, then the whole thing would accelerate downwards at exactly the same rate as the bar without the chain (neglecting air resistance).

Consider the chain in two parts - the stationary part and the moving part. As it falls, the moving part is transferring mass to the stationary part. Because energy and momentum have to be conserved, this decrease in moving mass has to result in an increase in velocity.

I found this paper which explains it properly: http://arxiv.org/pdf/1110.6035.pdf

I also found this video, which shows a different but similar phenomenon for rigid bodies: http://www.youtube.com/watch?v=BV7TPvk__kE

[Mr2Mike]

Just imagine the chain wrap around a pulley with a heavy mass attached to it, (i.e. like a block and tackle). For the mass on the pulley to accelerate at 9.8 m/s^2, the mass on the end of the chain would have to accelerate at twice this rate. Obviously in practice it doesn't attain twice the acceleration, but it is above 9.8 m/s^2

Another way to look at this is the loop in the chain only has to fall half the distance that the mass on the end does, yet they both reach the ground at the same time.

Edited by Mr2Mike on Monday 7th January 19:22

[V8LM]

Reminds me of this - http://www.youtube.com/watch?v=eCMmmEEyOO0

[XM5ER]

Ari said:

Got shown this the other day.

http://www.youtube.com/watch?v=X-QFAB0gEtE

Thing is, for all his explanation of the one of the chain hitting the ground sooner due to the chain "whipping the weight around thus making it accelerate faster", surely the real reason is that the one with the chain on hits the ground first simply because it has a blimmin great heavy chain attached to it so it's heavier?

Or am I just being thick? (Again).

The answer lies in the fact that the starting conditions are not the same. The chain is part of a total system including the bar and at the start, it's distance of travel is less than the bar alone. In addition you have a dynamic transfer of potential energy in the chain and bar as the system moves through the drop (one end of the chain remains attached at the top dont forget) thats why chain and bar (as a whole) appear to drop more slowly (remember the start position of the lower part of the chain).

Does that all make sense. The point is it is a deliberate trick to fool you into questioning basic principles. It would be a great video to show to kids in a class room to get them thinking about how various forces interact in a dynamic system though.