Originally posted by mtthwWell, yes, you've forseen my arguments.
I know what you're getting at. But it is possible to calculate, at least close enough. Engineers solve problems like that every day, using computational fluid mechanics and similar techniques - I used to know people who worked on this sort of problem. Obviously I'm not going to try and do that now!
But there is a flaw in the question posed in the first posting: "*Assume zero friction and assume the ball doesn't slow down or speed up after you've released it." Well, sand in movement has some viscosity. But if we treat the sand as a liquid (in order to neclect friction) then the solution is as described, the bridge will fall.
Originally posted by mtthwF=ma
I refer you to my earlier post. 🙂
No anti-gravity, no change in weight. Just a fluctuating acceleration due to a rotating non-homogeneous body . The downward force only equals the weight if there is no upwards acceleration.
Where is the force for your acceleration?
The sphere as a whole does not accelerate, there are no other forces acting on it. (Forget centrigfugal force ... it does not exist)
Originally posted by wolfgang59In a non-homogeneous ball the centre of mass is not necessarily at the centre. In which case, as it rolls the centre of mass will move up and down. So the vertical component of acceleration is non-zero. It is variable, with a zero average, but it is non-zero.
F=ma
Where is the force for your acceleration?
The sphere as a whole does not accelerate, there are no other forces acting on it. (Forget centrigfugal force ... it does not exist)
So, considering only vertical forces, where F is the force the bridge exerts on the ball (and vice-versa):
F - mg = ma(t)
Therefore F will vary with t as a varies.
If I've got time I'll solve a specific example later to demonstrate.
Originally posted by FabianFnasI'm not convinced by that - even a zero viscosity fluid will "slosh". But I suspect this is going further than the original question intended. I don't think we're really disagreeing about anything important.
But there is a flaw in the question posed in the first posting: "*Assume zero friction and assume the ball doesn't slow down or speed up after you've released it." Well, sand in movement has some viscosity. But if we treat the sand as a liquid (in order to neclect friction) then the solution is as described, the bridge will fall.
Originally posted by mtthwNo no, I think we agree on all essentials. I enjoy this debate really!
I'm not convinced by that - even a zero viscosity fluid will "slosh". But I suspect this is going further than the original question intended. I don't think we're really disagreeing about anything important.
And I also that you can carefully prepare an experiment in such a way that the bridge actually can hold. But in the general case, I don't think it will work.
Originally posted by uzlessThis anti-grav device will not work!
how much lift is generated by a rolling sphere? I would assume that the faster a sphere is rolled, the more lift it would generate....or does it produce a downforce?
A rolling sphere does NOT produce any force. Any apparent effects at a localised part of the sphere will be offset (equal & opposite) by effects at another part so that the WEIGHT of the sphere is CONSTANT on the bridge.
Originally posted by FabianFnasIt doesnt even work in the unreal-world of zero-friction, zero-resistance, perfect spheres or whatever else.
Would be nice if such a device actually worked.
Just rotate, with an imbalance, all the way to space, not needing any rockets anymore.
In real world, it doesn't work.
I am really surprised anyone thinks it does.
Originally posted by uzlessThe only thing that can change is the mass distribution in the ball. Since the ball touches the bridge at one point only this will not matter. The ball is 12 pounds.
This time we have a hollow sphere that weighs 10 pounds.
We pour 2 pound of sand inside the hollow sphere but do not fill up the entire void inside the sphere.
The bridge can only support 11 pounds before it collapses instantaneously.
Using the concept of centrifugal force (ie sand in a bucket) is there a speed that you can roll the sphere such that ...[text shortened]... as the sphere travels along the bridge?
Make any assumptions you need to that are realistic.
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Originally posted by kbaumen"Centrifugal force" is not a force. The only forces on the ball are gravity and normal force.
Are you sure?
Let's assume that we really reach a situation where to c(centrifugal)-force spreads the sand evenly against sphere's inside. Now at every moment a part of all sand is being pushed upwards by the c-force. Now the net weight is a vectorial sum of all forces involved, including the force pushing the sand upwards. Mass remains the same, but some it then cancels itself out and the weight remains.
Eh, gotta go learn some more dynamics.
Originally posted by FabianFnasBy that logic no physics is possible since we can't know all positions and velocities of every subatomic particle in any object. Physics is already an exercise in statistics - the statistical behavior of large numbers of small particles. Look at the Ideal Gas Law for example.
If you know every position and initial velocity of each grain (impossible), and... I stop there, I think you know where I'm getting at. Yes, by Newtonian mechanics, in theory, it is possible to calculate, but not in reality. So we have to rely on statistics. And then the anser is given above - the bridge will not hold.
Originally posted by uzlessA forward rolling sphere, if it were flying in the air, would curve downwards. To go upwards you need to make it spin in such a way that it would come rolling back to you if it hit the ground.
how much lift is generated by a rolling sphere? I would assume that the faster a sphere is rolled, the more lift it would generate....or does it produce a downforce?
Originally posted by AThousandYoungNo friction assumed, remember. This means no air restistance too...
A forward rolling sphere, if it were flying in the air, would curve downwards. To go upwards you need to make it spin in such a way that it would come rolling back to you if it hit the ground.