unbalanced rotating mass

The force has two. you list only one, another is mrw2sin(wt).

I guess the sentripetal should be centripetal force.

In this question, gravity cannt contribute to viberate. for its balanced by base. one is upward, one is downward, they are balanced. so no contribute.

the viberation force is really centripetal force when rotation. r is distance from equal mass center point to rotation center.

"In this question, gravity cannt contribute to viberate. for its balanced by base. one is upward, one is downward, they are balanced. so no contribute."

Do you mean that sometimes the gravity will contributy to a force up and sometimes down, so it's net effect is zero?

It is difficult to understand just what you are asking. I could make an educated guess and provide you with an answer and explanation, however, read the last paragraph. As CnPower said it is centripetal force. The centripetal force is not the force the mass applies to the structure, but the force the structure applies to the mass to keep it moving in a circle. Obviously we must apply Newton's Third Law and realize that our unbalanced object also exerts a force on the structure causing vibration. You seem to indicate that the structure is vertical, but is that actually the case? Some of my students have mistaken a drawing of a rotating object, such as a merry-go-round and assumed because of the orientation on the page it was vertical rather than a birds eye view of a horizontal rotating object. Unless you can explain more clearly (such as the actual language of the question and the artwork, if any), I suggest that you refer back to your instructor for the answer you seek. He knows what was intended in the question. Sometimes the questions themselves could have been written more clearly.

Also, just as reference; this forum is not here to do peoples homework for them (which is cheating) or help them cheat on a test. We prefer to think the best so we assume that this is an instructor provided sample or practice test. Your questions and concerns in that case are best referred to your instructor.

Do you mean that sometimes the gravity will contributy to a force up and sometimes down, so it's net effect is zero?

I wish you tell where a u from. and vote me and PhysicsProf a GA as a present

you need to read his thread carefully. and then you can understand what caused this viberation.

base-- structure base, or say machine base, seat, underlay, frame..... which install usually on the ground. we can also say it forced by ground, upward.

this must be a dynamics exercise.

This has nothing to do with a test or something, I am just trying to learn vibrations.

When I used the word vertical, I meant vertical oscilliations. Schetch of my problem:

http://bildr.no/view/383540

As you can see on the free body diagram. On the rotating mass, there is a force from the structure and there is gravity. It is said the the structure acts with the entire centripetal force on the mass, and then Newton's 3 law says that the mass excerts this force on the structure aswell. But if you say the structure acts with the enitre centripetal force, what then about the gravity that works on the mass? Because sometimes mg will be directed in the same direction as the centripetal force, and then the structure "will get help" to provide a big enough centripetal-force?

I am from Norway, I do ot think guests can vote here sorry.

You are right.

The total reaction on the supports is

= Mg + m.ω².r.cos(ωt)

Here the Mg = weight of the total rotating mass and will be constant

m = unbalance and r is the distance of CG from axis

This will be more like a AC signal superimposed over a DC signal, if you understand what I mean.

Thank you very much. I did not quite understand the electronic analogy. What do you mean by this?

cnpower: I can't find anywhere to vote, could you please provide the link you where you want me to vote?

Haha, college student, dont take it seriously. just kidding.

[the marking Rate is just the key]

sb's trope is not suitable, you can ignore it completely.

Current is a scalar whleas force is a vector. they cannt be analogised each other.

learn anylasis of force ans its constrain will make you to be a machine engineer.

you have a good condition than I had in university.

work hard and day day up ( chinglish)

In electrical engg, (as well in vibration) the main signal rides over a DC signal.

like below

Sorry for rough hand sketch but the values are corresponding to your parameters.

Now based on the individual amplitude and Mg, the signal can even reach the negative portion r remain in positive side only. Also in most of the cases may be negligible.

ie

Mg >> mw²rcoswt

In fact most of the designs under vibration, these are very critical aspects and also the frequency (wt)

no no no, the sketch is not bad. the only error is that it is a sine curve not cos curve. correct it pls.

I understand what you are talking about, but 1.the student is learning machine engineering.2.he will be more confused by this. especially scalar and vector relationship.

3. what is most important, dont mind what I said.

I cannt access to your web site. But I can imagine what you are saying. I can draw the picture.

If you concern the vertical viberation, the produce should be xsinwt, not xcos wt. while xcoswt is horizontal viberation.

this force acts on structure base which install on the ground or frame of machine. then the ground has a reaction force on the object, its just this force, cause machine viberation up and down.

the gravity has been balanced by the ground. so the sum of the gound force should be gravity + vertical partial of centripetal.

Now you will nderstand all. your instructor will tell you this as well. this is a very simple exercise. Other people who see this may think its too easy to answer.

guest can alos vote. no problem. student