# Tension in a rotating object

S

#### Splry00

##### Guest
Splry00 Asks: Tension in a rotating object
I'm somewhat stuck on this very simple question, but I can't convince myself of a satisfactory answer. I tried to look through the previous questions asked to avoid asking a duplicate I hope.

Any rotating object... say a rod in this case, sitting stationary in free space, when pushed at one end will cause a rotation about the center of mass. This means the pushed end and other end will move in opposite directions of each other.

The only way that the rod can transmit force down its length is through the interaction of each particle pushing/pulling on the next (tension perpendicular to the length) as it travels down the rod. Once you reach the center of mass, there is no induced rotation. How do the particles past the center of mass receive a push or pull at all if there is a point in the center which has no movement?

Additionally, how does this original force in the positive direction turn into movement in the negative direction past the center of mass? Essentially both of these questions can be answered by an explanation of how the particles inside an object behave when a torque is applied... which I cant find an answer to.

Edit: So I found this other post which asks a very similar question: Why does a rigid body rotate and not simply translate when pushed with an instantaneous force?

I feel that the corded particles explanation answer there is of the form I am imagining for this, but it seems that it doesn't explain the other side rotating backwards.

The particles in the rod near the push start to move forward, which "pull" on the neighboring particles via tension. Then at some point, this tension turns into compression, effectively pushing the particles past the center of mass downward.

Does anyone have a satisfactory internal particle explanation involving tension/compression caused by an initial force at one end which results in the final behavior of rotation of a rod? My understanding of rotational motion feels incomplete to me if I can't explain intuitively why something rotates when pushed off-center besides "if one side goes up, the other must go down".

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#### Custom loss function for regression

Filip Asks: Custom loss function for regression
I am trying to write a custom loss function for a machine learning regression task. What I want to accomplish is following:

• Reward higher preds, higher targets
• Punish higher preds, lower targets
• Ignore lower preds, lower targets
• Ignore lower preds, higher targets

All ideas are welcome, pseudo code or python code works good for me.

This is what I tried so far, it does not work so well I think it is because it does not take high targets into account (just high preds):

Code:
def mae_high(inp, targ):
inp, targ = flatten_check(inp, targ)
thresh = np.percentile(inp.detach().numpy(), 50)
return torch.abs(high_preds - high_targ).mean()

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#### How would I prove this?

alwayscurious Asks: How would I prove this?
if there is a polynomial-time, additive 80-approximation algorithm for the Minimum Vertex Cover problem, then P = NP.

What i am thinking is, since we already know that the Min vertex cover + 80 vertices which spans vertex cover can be solved in polynomial time and then we need to find V-80 vertices from V ( V is total number of vertices in vertex cover) which is also vertex cover. this can also be done in polynomial time. Therefore the Min. Vertex Cover Problem can be solved in polynomial time. Therefore, P=NP.

I feel like this is wrong explanation. Any help is appreciated.

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#### [Solved] how to avoid too curly contours in QGIS?

Usha Asks: how to avoid too curly contours in QGIS?
I had point data in DXF and generated contours using QGIS. The contours had smaller islands and the contours in itself are too curly. I smoothed the contours too and I could avoid those unwanted island by rejecting contours of certain length but how to avoid unwanted curliness in the single elevation contour. I am very new to QGIS and I don't know python too. I have attached an image of part of my contour here. I have marked the curly contours with blue while I expect them to be straight like marked in red.

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#### [Solved] SearchCursor in arcpy, help!

Hannah Asks: SearchCursor in arcpy, help!
I'm pretty new to programming and am blocked on the correct way to format a SearchCursor that I need. I've attached a subset of my data below. I'm looking for a way to get arcpy to print out a list of counties(Adams, Alamosa, etc.) for each species if the cell value is equal to a 4 or 5. I'm pretty stuck on the correct wording and syntax to use to get it to print out a list of field names based on an integer cell value as well as the species name. Any help would be great!

I also attached a screenshot of my current code.... it's definitely all sorts of wrong but I'm not sure where to go next.

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#### [Solved] Color gradient in raster symbology using ArcGIS Pro

I'm using ArcGIS Pro 2.8. I have a raster, shown below, whose data I would like to show in a smooth, detailed manner, like in the second raster shown. I recognize that the second raster has a much smaller range (the first one has a range of about 713-1363, while the second has a range of 1-353).

Is there a way for me to show the detail in the first raster?

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#### The car continues clicking when the key is turned, but the car would not start

• jeffrey
• Physics
• Replies: 0
jeffrey Asks: The car continues clicking when the key is turned, but the car would not start
The car continues clicking when the key is turned, but the car would not start..

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#### Suppose $G=\left\langle x, y, t|x^7=y^7=t^3=1, txt^{-1}=x^2, tyt^{-1}=y\right\rangle$. Show that $y\in Z(G)$.

Dick Grayson Asks: Suppose $G=\left\langle x, y, t|x^7=y^7=t^3=1, txt^{-1}=x^2, tyt^{-1}=y\right\rangle$. Show that $y\in Z(G)$.
The Problem: Suppose $G=\left\langle x, y, t|x^7=y^7=t^3=1, txt^{-1}=x^2, tyt^{-1}=y\right\rangle$. Show that $y\in Z(G)$.

My Attempt: Clearly $y$ commutes with $t$, so $y$ commutes with $t^2$ as well. Thus to show $y\in Z(G)$, it suffices to show that $y$ commutes with $x$. But I struggle to prove that $xy=yx$. It seems a rather silly place to get stuck-any HINT would be greatly appreciated.

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