# What does it mean that a rigid body is in equilibrium?

## Homework Statement:

What does it mean that an rigid body is in equilibrium?

## Relevant Equations:

##\sum F_x=0##
##\sum F_y=0##
## \sum \tau=0##
So I have always been thinking that equilibrium means that an object is not moving or having constant acceleration. On a webside they said: " A rigid body is in equilibrium when it is not undergoing a change in rotational or translational motion. " To me it sounds like the object then must not be moving or rotating. But in several problems I come across for example this one:

The object is moving and is undergoing a rotational motion . I then thought I could not use the equations above. So it makes me a bit confussed and I find it hard to identify when I'm dealing with a rigid-body equlibrium problem.

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.Scott
Homework Helper
" A rigid body is in equilibrium when it is not undergoing a change in rotational or translational motion."
That doesn't mean it isn't moving. Only that the motion is not changing.

Kolika28
That means that the body is not accelerating or decelerating, neither in rectilinear or circular movement; therefore, all forces or moments acting on it, if any, cancell each other.
If one force or moment prevails among the others, then, the mass of the body will change its linear or/and angular velocity.

Kolika28
Ohh, I see, that make sense. Thank you so much both of you!