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Accurate definition of equilibrium

As a part of Mechanics 1 revision, I have been pondering the exact definition of mechanical equilibrium.

The textbook says: there is no resultant force and hence it will remain at rest.

Now what I wish to clarify is whether this definition also includes the fact that an object in uniform motion (ie not at rest) also has no resultant force?
After some internet searches I cannot find anywhere that explicity says, yes the definition covers both objects at rest, and also in uniform motion.

I assume the equilibrium definition applies specificially to objects at rest, since the term is most commonly applied to statics (as opposed to dynamics) problems.

Thank you in advance.:smile:
Thank you, the static and dynamic mechanical equilibrium terms are helpful.
Original post by Choochoo_baloo
Thank you, the static and dynamic mechanical equilibrium terms are helpful.


Out of interest, what did you learn about Newton's Laws of motion?
Reply 3
Avatar for krisshP
krisshP
14
Original post by Choochoo_baloo
As a part of Mechanics 1 revision, I have been pondering the exact definition of mechanical equilibrium.

The textbook says: there is no resultant force and hence it will remain at rest.

Now what I wish to clarify is whether this definition also includes the fact that an object in uniform motion (ie not at rest) also has no resultant force?
After some internet searches I cannot find anywhere that explicity says, yes the definition covers both objects at rest, and also in uniform motion.

I assume the equilibrium definition applies specificially to objects at rest, since the term is most commonly applied to statics (as opposed to dynamics) problems.

Thank you in advance.:smile:

WRONG! Who makes such awful textbook definitions?

Look at Newton's first law. No resultant force on the object means no acceleration, so there's the object either is in rest OR has constant velocity. The key thing here is that a resultant force means acceleration.

Now equilibrium is a state of balance where all forces are balanced, so there is no resultant force. Hence the object either remains in rest or has constant velocity.

I understand what you mean though. For example it is hard to imagine say the plank involved in moments having constant velocity like say 23ms^-1 since it would just move away from its support, weird:eek:. Here I think it is reasonable to just think that the plank is at rest.
(edited 10 years ago)
If you're going to do the thing properly, you also need to say that the forces have a zero resultant moment about any point.
Reply 5
Avatar for 1gustavo
1gustavo
Original post by krisshP
WRONG! Who makes such awful textbook definitions?


It's not wrong.
You have merely extended the definition with the text you have used.
It's highly likely that the definition quotes was from the chapter on statics.
Reply 6
Avatar for TenOfThem
TenOfThem
18
Original post by 1gustavo
It's not wrong.
You have merely extended the definition with the text you have used.
It's highly likely that the definition quotes was from the chapter on statics.


Indeed ... this looks like part of the definition from page 115 of the Edexcel M1 text book ... key points at the end of the Statics chapter

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