Wm2d Question

[FunWithGravity2]

Hello i have an issue with wm2d, i have the demo

I have seen many instances where the program showed a working model but it did not work in real life for the obvious reason, friction air etc. But i have the opposite problem, it works in real life but not in the program. At least not with the air on, i am not sure where the friction is.

I have a large circle with a central pivot. I have a single item attached to the circle with another pivot, i have a stop attached with a fixed attachment, that is all so i am sure its not because its to complicated.


When i run it as built it will behave just as in my real world, make one complete turn with the weight ending up higher than the start, But as soon as i turn on the air it will not behave like in the real world.

Has anyone had issues with simple mechs not duplicating real world in WM2d?

Can anyone suggest where and how to set the friction and air to correctly mimic real world as close as possible.?

Thanks
Dave

[Fletcher]

Jeez dave - pictures help - a couple of things - if the stop is fixed to the large circle then you may need to add a dummy one 180 degrees apart to balance it out so you are really only looking at the effect of the pivoted weight - even a small mass stop can effect things & make it appear to lift the pivoted weight higher than it started - btw mass is changed in the >window>properties box [you could set a low mass for the stop e.g. 0.0001kgs]

Next I suggest you also use the windows menu > appearance & select no pattern - this will make the circle transparent - then go to >view> show system Center of Mass - zoom right in on the system CoM & run the sim - watch closely to see if the CoM moves & where it ends up.

Air friction is set by default - you do have the ability to alter it by choosing between low speed & high speed - low speed is fine for these types of experiments - you can put your own figures in if you want to - N.B. high speed uses V^2 whilst low speed uses V which is fine for most low rpms.

Pin frictions can be altered but it depends on the diameter etc to get the correct coefficient so usually I just leave it at program default for those diameters.

[FunWithGravity2]

Thank you Fletcher, I know that asking without the picture is rude so i do apologize. I did no consider the weight of the pin, and now i know why i couldn't see the COM. LOL


I really appreciate your input.

dave

[FunWithGravity2]

Hello Fletcher,

Tried all that, and learned a little more about the program, its kinda fun but i think it may have a flaw. I do not think it is correctly calculating the system center of mass. Has this been discussed before and if so could you point out a thread. I tried searching "wm2d center of mass" and got a tone of posts where it was mentioned but not discussed at length as to whether anyone thought it was doing the calculation correctly.

I know many have had sims that work and then the real world did not match, and i have one were the real world works but the sim refuses. I have changed all mass variables and sizes just to be sure. But still no go with air on low.


I did a couple sample Sims on wm2d with "swinging" weights and it appears that the program just averages the COM of the objects and doesn't take into account any "weightless qualities" i know nothing is ever weightless, but in regards to the reference frame and the system COM it can be. Or am i completely crazy now.


Thanks for any answers.
Crazy dave

[Fletcher]

Yep, you're crazy now, to not put up a picture & expect any answers that aren't ball park guesses from me.

Every individual component you make on your worksheet has an individual CoM - bring up >window>appearance>track CoM - then go to >window>geometry>CoM Offset - move the CoM to where it would be if not uniform etc for each piece.

This is tweaking the sim towards reality - the system CoM takes account of EVERY component on your page [average] - remove all unnecessary componentry - other stuff reduce the mass as much as practical so that it has little effect on system CoM, if it must stay on the worksheet.

No, there is no flaw in the program, just your lack of familiarity with it - it does require a certain amount of logical order to doing things with the program & evaluating what you are seeing.

[KAS]

I once designed a wheel on WM2D that worked perfectly. It fooled me and just about everyone else here. It had us all scratching their heads while we watched it spin.

I was eventually told the flaw was down to fixed pins; you know, the little square ones.

Now, I only use the circular pivot type pins, adding 2 of them to act as a fixed pin.

A member of another forum contacted me with similar problems.
After I changed them all, the wheel (naturally) wouldn't work.

Kas

[FunWithGravity2]

Sorry and thanks for your help, i think it must be a misunderstanding that i am having with "system COM" i was thinking in a circle with weight attached that this would be tracking the moment of inertia on the circle and showing where all the weights were exerting their mass. Now i realize it is just a an average of the masses within the system and not necessarily indicative of the systems moment of inertia.


Sorry


Dave

[Jon J Hutton]

I am trying to code the box that says "active when" in wm2d. Please Help!!!

When object 1 is close to object 2, I want to turn on the electrostate to on and off when object 1 is close to object 3....

Is this possible?

Jon

[Ed]

Hi Jon,

It's not possible to use a formula to turn on/off the main electrostatic setting, but you could put a formula in the Charge field for each body:

if(body[1].p.x < body[2].p.x, 2.998e+005, 0)

I hope that helps. Let me know if you have any other questions.