# Homework Help: Static and Dynamic Equilibrium

1. Jan 7, 2006

### jrm2002

Static and Dynamic Equilibrium!!

What is the difference between static equilibrium and dynamic equilibrium?

2. Jan 7, 2006

### Pyrrhus

Well, Mechanical equilibrium exist when Newton's 1st Law is true on our system of interest. Just remember that the first law mentions constant velocity, it could be 0 or not.

3. Jan 7, 2006

### Hootenanny

Staff Emeritus
Static equilibrium is when all forces acting on an object are balanced (i.e. there is no resultant forces) and the object is not in motion releative to the plane of reference, in otherwords the object does not have a velocity.

Dynamic equilibrium is when all the forces acting on an object are balanced bu the object it moving, it has a veleocity.

4. Jan 8, 2006

### jrm2002

How can it be said that all the forces are balanced?The body/object is moving because there is a resultant force acting right?Then how can you say that all forces are balanced?
Plz help!!

5. Jan 8, 2006

### Hootenanny

Staff Emeritus
No. If a resultant force is acting on the object, then the object will accelerate. F = ma, therefore if F = 0 there must be no acceleration. Newton's first law sates that a body will remain at rest or continue is a straight line with a constant velocity unless it is compelled to change by an external force.

Last edited: Jan 8, 2006
6. Jan 9, 2006

### vaishakh

Mechanical equilibrium states that the net external force acting on an object is 0. This according to the second law of Newton implies that the object cannot accelerate. However we are not certian about the velocity of the object. If the object had some velocity during the instant the net force on it became 0, then it will also not deccelearate unless a force is applied against its motion. Thus the object will continue its motion. In short its velocity remains the same. You can be sure that objects in mechanical equilibrium, have the same state in all inertial reference frames. Moreover two object in mechanical equilibrium in an inertial reference frame are in Mechanical equilibrium with respect to each other. But the state of Dynamic and static equilibrium need not be same in all inertial reference frame. Objects which are at rest in a given frame is called to be in Static equilibrium. Object which are in uniform motion with a constant nonzero velocity is saidto be in Dynamic equilibrium.

7. Aug 28, 2007

### Jaeheung Park

Is there any good reference explaining this "dynamic equilibrium"?

I now understand the difference between static and dynamic equilibrium.

Is there any reference book for that?

Thanks

8. Jan 12, 2012

### DarkMatterHol

Re: Static and Dynamic Equilibrium!!

I had it explained to me this way:

Static equilibrium is a case where a system in equilibrium will return to equilibrium when an outside force pushes a system out of equilibrium. Dynamic equilibrium is a case where a system in equilibrium will not return to equilibrium once pushed out of equilibrium.

They provided the following example:
Take a basic coffee cup on it's side. If you balance a marble inside the cup at the equilibrium point, at the horizontally flat point of the arc, it will return to the equilibrium point on it's own when you push it away from there.

If you balance a marble outside the cup at the equilibrium point, at the horizontally flat point of the arc, it will not return to the equilibrium point on it's own when you push it away from there - instead it will fall off.

As i recall this was related to a discussion of negative v positive feedback. Natural systems have elements of positive and negative feedback that determine the types of equilibrium that can result.

[for what it's worth - feel free to correct me as required]