- #36
Jediknight
- 41
- 3
so this posts a month old, I am probably good to propose some answers, a(max)=ω^2(A)=g at the minimum frequency to make block leave plank, so just algebra from there right
Yes.Jediknight said:so this posts a month old, I am probably good to propose some answers, a(max)=ω^2(A)=g at the minimum frequency to make block leave plank, so just algebra from there right
It might depend on the conventions adopted for the positive direction and what 'g' means. If we take up as positive and g as a signed value (i.e. its value will be negative) then the equation is mg+N=-mω2x. Other conventions can lead to other forms. Since N will be set to zero, it may be that the equation was simply wrong in a way that didn't matter.andyrk said:While pondering on this problem once again, I got some more doubts with it. Why is mg - N = mω2x and not: mg - N = -mω2x, since acceleration is
-ω2x and not ω2x?
The convention is that up is positive and down is negative.haruspex said:It might depend on the conventions adopted for the positive direction and what 'g' means. If we take up as positive and g as a signed value (i.e. its value will be negative) then the equation is mg+N=-mω2x. Other conventions can lead to other forms. Since N will be set to zero, it may be that the equation was simply wrong in a way that didn't matter.
How do we know that the downward acceleration of the plank would be greater than the block? We just saw that g = ##\omega^2A##. And also, the downward acceleration of the block at the topmost point is ##\omega^2A##. So shouldn't the two be same?haruspex said:No, the other way around. When the maximum downward acceleration available to the block (g) is less than the downward acceleration required to stay in contact (which can be up to ##\omega^2A##) the block will lose contact with the plank, no?
We don't know the plank's downward acceleration is greater than the block's. I said if it is ever greater.andyrk said:How do we know that the downward acceleration of the plank would be greater than the block? We just saw that g = ##\omega^2A##. And also, the downward acceleration of the block at the topmost point is ##\omega^2A##. So shouldn't the two be same?
By two I mean the downward acceleration of the block and the plank at the topmost point. If they are the same, then why would the block leave contact? Now you are saying if they are same. But what I am trying to say that they have to be same, there's no if associated with it, it just has to be the same.haruspex said:I don't know what you mean about "the two" being the same. What two?
The value of g is fixed. Give the plank enough amplitude and its acceleration at the top of the cycle will exceed g. The block will have less downward acceleration than the plank and lose contact with it. The normal force cannot be negative.andyrk said:By two I mean the downward acceleration of the block and the plank at the topmost point. If they are the same, then why would the block leave contact? Now you are saying if they are same. But what I am trying to say that they have to be same, there's no if associated with it, it just has to be the same.
The acceleration of the plank at the top is ω2A, right? And the acceleration of the block is g, right? And we also proved that g = ω2A (by putting N = 0 in the force equation of the block). So, we just showed that the accelerations are same at the topmost point and not different. But wait. Had they been the same, then the block wouldn't have left contact and hence N wouldn't have been 0. So what am I doing wrong here?haruspex said:The value of g is fixed. Give the plank enough amplitude and its acceleration at the top of the cycle will exceed g. The block will have less downward acceleration than the plank and lose contact with it. The normal force cannot be negative.
We could only write N=0 in that equation on the basis that the block was almost leaving the plank but not quite. If the block leaves the plank then N will still be zero, but the equation we plugged it into is no longe valid. The equation assumes they remain in contact.andyrk said:The acceleration of the plank at the top is ω2A, right? And the acceleration of the block is g, right? And we also proved that g = ω2A (by putting N = 0 in the force equation of the block). So, we just showed that the accelerations are same at the topmost point and not different. But wait. Had they been the same, then the block wouldn't have left contact and hence N wouldn't have been 0. So what am I doing wrong here?
At the top position, the downward acceleration of the plank will always be ω2A.andyrk said:I am unable to make any sense of what you are saying. Are you then saying that the downward acceleration of the block is less than g and that of the plank is g? If block leaves contact at the top and its acceleration is g, then does that mean that the acceleration of the plank is greater than g, i.e. greater than ω2A? But how is this possible??
But we just found out that ω2A = g by putting N = 0 in the equation mg - N = mω2A. So isn't ω2A = g at the topmost position? Which means that min{ω2A, g} = g or ω2A since they are one and the same thing? So that means that they should not leave contact but that is not the case as it is clearly mentioned that the block leaves contact. This is what is confusing me.haruspex said:The downward acceleration of the block at that position will be min{ω2A, g}
As I wrote, that equation is only valid by assuming they stay in contact.andyrk said:But we just found out that ω2A = g by putting N = 0 in the equation mg - N = mω2A.
How can they stay in contact if N = 0?haruspex said:As I wrote, that equation is only valid by assuming they stay in contact.
That's the boundary case. Whether you consider that to be contact or not is somewhat a matter of taste. I would have said it could be that that N=0 and yet there is no separation between them, but it really doesn't matter. In the same way, you can argue that no two real physical quantities are ever exactly equal. But they can be near enough equal that you can't measure the difference, so who cares?andyrk said:How can they stay in contact if N = 0?
... and in this limiting case, N = 0 for only an instant, when the plank is at it's very highest position. (whatever an instant is physically)haruspex said:That's the boundary case. Whether you consider that to be contact or not is somewhat a matter of taste. I would have said it could be that that N=0 and yet there is no separation between them, but it really doesn't matter. In the same way, you can argue that no two real physical quantities are ever exactly equal. But they can be near enough equal that you can't measure the difference, so who cares?
What I am feeling is that if g = ω2A and acceleration of plank > ω2A then that means ω2A of plank is different from that of the block.haruspex said:That's the boundary case. Whether you consider that to be contact or not is somewhat a matter of taste. I would have said it could be that that N=0 and yet there is no separation between them, but it really doesn't matter. In the same way, you can argue that no two real physical quantities are ever exactly equal. But they can be near enough equal that you can't measure the difference, so who cares?
The acceleration of the plank cannot exceed ω2A.andyrk said:What I am feeling is that if g = ω2A and acceleration of plank > ω2A then that means ω2A of plank is different from that of the block.
And if acceleration of block = ω2A, then I don't see how could they get separated? But we found out that acceleration of the block ω2A by assuming they get separated, which is just very ironical. How could two object which are accelerating at the same rate ever get separated?haruspex said:The acceleration of the plank cannot exceed ω2A.
No, by assuming that it was right on the boundary between separating and not separating.andyrk said:But we found out that acceleration of the block ω2A by assuming they get separated,
So you mean to say that acceleration of the block is ω2A when it is just about to separate but hasn't yet separated? Also called as "Just about to break off"? But then the acceleration of the block is ω2x at all times where x is the distance from mean position. Why then, is the block not on the verge of breaking off at all these times?haruspex said:No, by assuming that it was right on the boundary between separating and not separating.
No, I didn't say that. The minimum normal force, Nmin, is at the top of the movement, agreed? When Nmin>0 and the plank is at the top of its movement the block will have acceleration ω2A, and it will be less than g. Suppose we gradually increase A somehow. As ω2A gets closer to g, Nmin will decrease. At the point where ω2A equals g, Nmin will be zero and the block is on the verge of losing contact. The slightest further increase in A will cause the block to lose contact.andyrk said:So you mean to say that acceleration of the block is ω2A when it is just about to separate but hasn't yet separated? Also called as "Just about to break off"?
So when you say "we gradually increase A somehow" you mean increasing the amplitude of the SHM that the whole system (plank+block) is executing? So the break-off point is reached at a certain amplitude? But how can you increase A since it is fixed and if you change A you change the equation of SHM, which I don't think can be done.haruspex said:No, I didn't say that. The minimum normal force, Nmin, is at the top of the movement, agreed? When Nmin>0 and the plank is at the top of its movement the block will have acceleration ω2A, and it will be less than g. Suppose we gradually increase A somehow. As ω2A gets closer to g, Nmin will decrease. At the point where ω2A equals g, Nmin will be zero and the block is on the verge of losing contact. The slightest further increase in A will cause the block to lose contact.
I don't care how we increase it. Maybe we just run hundreds of different experiments, each with a slightly higher A than the one before.andyrk said:So when you say "we gradually increase A somehow" you mean increasing the amplitude of the SHM that the whole system (plank+block) is executing? So the break-off point is reached at a certain amplitude? But how can you increase A since it is fixed?
The equation of SHM is fixed for the problem and so is the amplitude. So how will you go about proving that the block leaves contact/is about to leave contact with the plank even though their accelerations are same at the highest point? This problem has become so complicated.haruspex said:I don't care how we increase it. Maybe we just run hundreds of different experiments, each with a slightly higher A than the one before.
How about rather than increasing A, we see start with small enough ω so that the block maintains contact with the plank, then similar to what haruspex has suggested, we run successive experiments each time with slightly larger ω until we find the minimum value of ω that results in the block losing contact with the plank. Certainly you can't object to increasing ω in this fashion.andyrk said:So when you say "we gradually increase A somehow" you mean increasing the amplitude of the SHM that the whole system (plank+block) is executing? So the break-off point is reached at a certain amplitude? But how can you increase A since it is fixed ...
Rightly said. So the block would leave contact with the plank at a specific ω. But when ω2A = g, then the block is on the verge of losing contact, right? Any slight increase in ω would make the block to lose contact with the plank. But right now the acceleration of the block equals that of the plank, right? So doesn't that happen at all instants, even when the plank is not at its highest point? Or does it not because N is not 0 at all other instants except for the topmost point?SammyS said:How about rather than increasing A, we see start with small enough ω so that the block maintains contact with the plank, then similar to what haruspex has suggested, we run successive experiments each time with slightly larger ω until we find the minimum value of ω that results in the block losing contact with the plank. Certainly you can't object to increasing ω in this fashion.
So finally, N = 0 has nothing to do with different accelerations of the block and the plank. They are the same at the topmost point and all points, yet N = 0 because of the algebraic reasoning we provided earlier. Is that correct?SammyS said:How about rather than increasing A, we see start with small enough ω so that the block maintains contact with the plank, then similar to what haruspex has suggested, we run successive experiments each time with slightly larger ω until we find the minimum value of ω that results in the block losing contact with the plank. Certainly you can't object to increasing ω in this fashion.
Many are here.andyrk said:Anybody there?
By that I mean when the plank and the block are the extreme position of the SHM, i.e. A distance from the mean position.SammyS said:In post # 65, what do you mean by "right now" when you say, "But right now the acceleration ..." .
If the block maintains contact with the plank, then they have the same acceleration at all positions.andyrk said:By that I mean when the plank and the block are the extreme position of the SHM, i.e. A distance from the mean position.