Just a quick one, Have I got my Mass/Spring/Damper model correct?

[Jowin86]

Yesssssss!

am i right about the other side? In which case the eom would be:

mx+kx-kYoSinWt=-cx (with the appropriate dots above the x's)

 

[Jowin86]

ignore the previous post I hadn't read your new post.


erm, if down is + then i'd say +. so it'd be:

mx+kx+kYosinWt=-cx

 

[rude man]

Yesssssss!

am i right about the other side? In which case the eom would be:

mx+kx-kYoSinWt=-cx (with the appropriate dots above the x's)

The sign of Ky0sin(wt) affects the phasing of x''/y''. +y(t) imparts a force to the spring which in turn tends to make M go positive. So +Ky is a positive force on M and that's the equation we're concerned with: sum of forces on M = M times M's acceleration.

 

[Jowin86]

mx+kx+kYosin(Wt)=-cx is the eom then?

It seems to make sense that way. Our lecturer didn't really teach us how to do this, he just did about 2 example and never said how he got the eom. :-/ guess that's uni for you hey.

Thank you for your guidance :)

Now i just have to sort the two odd values for phase lagg :'(

 

[rude man]

mx+kx+kYosin(Wt)=-cx is the eom then?

It seems to make sense that way. Our lecturer didn't really teach us how to do this, he just did about 2 example and never said how he got the eom. :-/ guess that's uni for you hey.

Thank you for your guidance :)

Now i just have to sort the two odd values for phase lagg :'(

No, the sign on Ky0sin(wt) is - if you put it on the left side of the eom.

Write the equation as mx'' = Ky0sin(wt) - cx' - Kx so it looks like ma = F. Then try to understand why each term's sign is + or -. Makes all the difference! For example, if either the cx' or Kx term were poitive per the above eom you'd get an exploding system! Those terms would represent what engineers call positive feedback which means the bigger the input, the bigger the output, feeding on itself until boom!

 

[Jowin86]

of course, i see what you're saying now! Thanks!

 

[Jowin86]

Could you advise me on which equation for MF you would use to plot the theory frequency response for this system given this equation of motion? there's 3 I'm trying to decide between, the bottom is always the same but on top it could be either 1, √1+(2ζr^2) , or r^2 .

Following notes i seem to be ending up at √1-(2ζr^2) on top but if i plot it with r^2 on top it seems to follow my practical experiment results better so I'm pretty confused now and can't find any help in textbooks because they talk about either relative motion or absolute motion where as this system is kind of a mix :-/