# If a particle is in equilibrium is the initial speed always 0?

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I get that there's no acceleration if the particle is in equilibrium and that the particle is either stationary or moving at a constant velocity.

Am I wrong in saying neither of those prove the initial speed must be 0?

Essentially the question I'm referring to the particle is originally in equilibrium before a force is removed. I've worked out the acceleration, am given that t=3, and I'm meant to find out the distance travelled by the particle, so of course I need either the initial speed, final velocity. The answer says u=0 which prompted me to ask this question.

Am I wrong in saying neither of those prove the initial speed must be 0?

Essentially the question I'm referring to the particle is originally in equilibrium before a force is removed. I've worked out the acceleration, am given that t=3, and I'm meant to find out the distance travelled by the particle, so of course I need either the initial speed, final velocity. The answer says u=0 which prompted me to ask this question.

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SYEPHEN17

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#2

You are exactly right. Can be constant velocity or at rest. Hard to give a sure answer here for this specific question. Check the wording to look for any hints

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Notnek

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#3

(Original post by

I get that there's no acceleration if the particle is in equilibrium and that the particle is either stationary or moving at a constant velocity.

Am I wrong in saying neither of those prove the initial speed must be 0?

Essentially the question I'm referring to the particle is originally in equilibrium before a force is removed. I've worked out the acceleration, am given that t=3, and I'm meant to find out the distance travelled by the particle, so of course I need either the initial speed, final velocity. The answer says u=0 which prompted me to ask this question.

**dont know it**)I get that there's no acceleration if the particle is in equilibrium and that the particle is either stationary or moving at a constant velocity.

Am I wrong in saying neither of those prove the initial speed must be 0?

Essentially the question I'm referring to the particle is originally in equilibrium before a force is removed. I've worked out the acceleration, am given that t=3, and I'm meant to find out the distance travelled by the particle, so of course I need either the initial speed, final velocity. The answer says u=0 which prompted me to ask this question.

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RichE

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**dont know it**)

I get that there's no acceleration if the particle is in equilibrium and that the particle is either stationary or moving at a constant velocity.

Am I wrong in saying neither of those prove the initial speed must be 0?

Essentially the question I'm referring to the particle is originally in equilibrium before a force is removed. I've worked out the acceleration, am given that t=3, and I'm meant to find out the distance travelled by the particle, so of course I need either the initial speed, final velocity. The answer says u=0 which prompted me to ask this question.

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#5

(Original post by

You are exactly right. Can be constant velocity or at rest. Hard to give a sure answer here for this specific question. Check the wording to look for any hints

**SYEPHEN17**)You are exactly right. Can be constant velocity or at rest. Hard to give a sure answer here for this specific question. Check the wording to look for any hints

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dont know it

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#6

(Original post by

Please post the full question.

**Notnek**)Please post the full question.

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Notnek

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#7

(Original post by

Think the answer I was looking for was in fact in the question, it asks to find the distance travelled during the first 3 seconds of motion, so we know that u=0. Correct me if I'm wrong

**dont know it**)Think the answer I was looking for was in fact in the question, it asks to find the distance travelled during the first 3 seconds of motion, so we know that u=0. Correct me if I'm wrong

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#8

(Original post by

I would normally understand "equilibrium" to mean that the particle is in the same place for all time, so necessarily velocity and acceleration are zero.

**RichE**)I would normally understand "equilibrium" to mean that the particle is in the same place for all time, so necessarily velocity and acceleration are zero.

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#9

(Original post by

Can you please post the question in full i.e. post the exact wording that you're seeing in e.g. your textbook.

**Notnek**)Can you please post the question in full i.e. post the exact wording that you're seeing in e.g. your textbook.

In this question i and j represent the unit vectors east and north respectively.

The forces (3ai + 4bj) N, (5bi + 2aj) N and (-15i-18j)N act on a particle of mass 2kg which is in equilibrium.

a)find the values of a and b.

b) The force (-15i-18j) is removed. Work out:

i)The magnitude and direction of the resulting acceleration of the particle.

ii)The distance travelled by the particle in the first 3 seconds of its motion.

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oShahpo

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#10

Equilibrium in this context normally refers to equilibrium of forces. At which point, there is no net force. This means that the particle is at a constant velocity (which could be zero, implying particle is at rest.) So in this sense, you are right, equilibria don't necessitate rest. The question should have been clearer, good spot.

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RichE

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#11

(Original post by

So you're saying that for you, non-zero constant speed would not mean that a particle was in equilibrium? Or am I misunderstanding you?

**Notnek**)So you're saying that for you, non-zero constant speed would not mean that a particle was in equilibrium? Or am I misunderstanding you?

https://en.wikipedia.org/wiki/Equilibrium_point

but I can imagine the word may get used in different ways.

PS now reading the given question, it doesn't matter whether the particle is in motion or not.

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Notnek

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#12

(Original post by

Yes, that's how I'd usually use the term. I guess in line with

https://en.wikipedia.org/wiki/Equilibrium_point

but I can imagine the word may get used in different ways.

**RichE**)Yes, that's how I'd usually use the term. I guess in line with

https://en.wikipedia.org/wiki/Equilibrium_point

but I can imagine the word may get used in different ways.

although I can't find this definition anywhere else outside of A Level which is a bit odd.

I'm pretty sure that an exam question would never be ambiguous and just say equilibrium if the body is at rest - normally questions say "rests in equilibrium".

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username3262012

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#13

Im not 100% sure but if a particle is in equilibrium isnt it traveling at 10x the speed of light? (3x10>4 km>-1s^-1) ?

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oShahpo

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(Original post by

Yes, that's how I'd usually use the term. I guess in line with

https://en.wikipedia.org/wiki/Equilibrium_point

but I can imagine the word may get used in different ways.

PS now reading the given question, it doesn't matter whether the particle is in motion or not.

**RichE**)Yes, that's how I'd usually use the term. I guess in line with

https://en.wikipedia.org/wiki/Equilibrium_point

but I can imagine the word may get used in different ways.

PS now reading the given question, it doesn't matter whether the particle is in motion or not.

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RichE

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#15

(Original post by

Surely it does? At initial speed of 1 million ms^-1 and acceleration of whatever, the particle will come cover a much longer distance in 3 seconds than at initial speed of 0.

**oShahpo**)Surely it does? At initial speed of 1 million ms^-1 and acceleration of whatever, the particle will come cover a much longer distance in 3 seconds than at initial speed of 0.

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SYEPHEN17

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(Original post by

The question says to give the distance after the first 3 seconds of motion. That means we assume it was stationary when in equilibrium right, and so we know from that u must be 0.

**dont know it**)The question says to give the distance after the first 3 seconds of motion. That means we assume it was stationary when in equilibrium right, and so we know from that u must be 0.

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SYEPHEN17

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(Original post by

Im not 100% sure but if a particle is in equilibrium isnt it traveling at 10x the speed of light? (3x10>4 km>-1s^-1) ?

**tome123**)Im not 100% sure but if a particle is in equilibrium isnt it traveling at 10x the speed of light? (3x10>4 km>-1s^-1) ?

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#18

(Original post by

I would assume that it was at rest, as ‘first three seconds of motion’ implies that at t=0 it was not moving, and beginning motion.

**SYEPHEN17**)I would assume that it was at rest, as ‘first three seconds of motion’ implies that at t=0 it was not moving, and beginning motion.

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#19

(Original post by

This agrees with the Edexcel (well at least the Edexcel M1 textbook) definition:

although I can't find this definition anywhere else outside of A Level which is a bit odd.

I'm pretty sure that an exam question would never be ambiguous and just say equilibrium if the body is at rest - normally questions say "rests in equilibrium".

**Notnek**)This agrees with the Edexcel (well at least the Edexcel M1 textbook) definition:

although I can't find this definition anywhere else outside of A Level which is a bit odd.

I'm pretty sure that an exam question would never be ambiguous and just say equilibrium if the body is at rest - normally questions say "rests in equilibrium".

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