# M3: work done against friction equation?

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#1
Initial Energy (PE - either, EPE/GPE, etc) = energy Lost + or - WD against friction?
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4 years ago
#2
it would be plus. initial energy is the total energy in the system and you need to conserve energy so.
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#3
(Original post by BTAnonymous)
it would be plus. initial energy is the total energy in the system and you need to conserve energy so.
ok
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4 years ago
#4
(Original post by Maths&physics)
Initial Energy (PE - either, EPE/GPE, etc) = energy Lost + or - WD against friction?
Total energy at start = (Total energy from conservative forces) + (Energy due to friction)

Energy from conservative forces is more or less the exact same one what you use for the LHS; the EPE, GPE, KE, etc...
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#5
(Original post by RDKGames)
Total energy at start = (Total energy from conservative forces) - (Energy due to friction)

Energy from conservative forces is more or less the exact same one what you use for the LHS; the EPE, GPE, KE, etc...
(Original post by BTAnonymous)
it would be plus. initial energy is the total energy in the system and you need to conserve energy so.
is it a plus of minus. it makes sence that its a + because: total energy = (energy gained, like GPE, EPE, KE) + (energy taken by friction)???

or does it depend on direction/relative to the initial energy at the start?
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4 years ago
#6
[QUOTE=Maths
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4 years ago
#7
(Original post by Maths&amp;physics)
is it a plus of minus. it makes sence that its a + because: total energy = (energy gained, like GPE, EPE, KE) + (energy taken by friction)???
I'd give RDK the credibility. I don't even do M3 lmao
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#8
(Original post by BTAnonymous)
Maths
pardon?
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#9
(Original post by BTAnonymous)
I'd give RDK the credibility. I don't even do M3 lmao
(Original post by RDKGames)
Total energy at start = (Total energy from conservative forces) - (Energy due to friction)

Energy from conservative forces is more or less the exact same one what you use for the LHS; the EPE, GPE, KE, etc...
I thought it would depend on circumstances?
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4 years ago
#10
(Original post by Maths&physics)
I thought it would depend on circumstances?
Total initial energy= Total final energy + Energy lost due to friction
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#11
Total initial energy= Total final energy + Energy lost due to friction
that makes sense, as the initial energy is shared. would there be a case when the total final energy is greater than the initial energy, but it balances out because - work done against friction?
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4 years ago
#12
(Original post by Maths&physics)
that makes sense, as the initial energy is shared. would there be a case when the total final energy is greater than the initial energy, but it balances out because - work done against friction?
Never. Unless work is done on the system by an external force the final cannot be greater than initial due to conservation of energy, as you cannot get energy out of nothing.
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#13
Never. Unless work is done on the system by an external force the final cannot be greater than initial due to conservation of energy, as you cannot get energy out of nothing.
that makes sense. thank you.
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4 years ago
#14
(Original post by Maths&physics)
that makes sense. thank you.
I suggest

Mechanical energy at start (KE, Grav. PE,Elastic PE) + Work done on system (by engine, cyclist, someone pulling) - work done against friction or resistance = mechanical energy at the end
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#15
(Original post by tiny hobbit)
I suggest

Mechanical energy at start (KE, Grav. PE,Elastic PE) + Work done on system (by engine, cyclist, someone pulling) - work done against friction or resistance = mechanical energy at the end
thank you. how does yours differ from the above?
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4 years ago
#16
(Original post by Maths&physics)
thank you. how does yours differ from the above?
Two ways, one of which doesn't matter:

I've put in the option of energy being put into the system by an engine, cyclist etc.

My brain finds it more obvious to think of energy being taken out of the system by doing work against friction etc. This is why I have it as a minus on the left hand side. As you've realised, this is exactly the same as adding it on the right hand side.
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#17
(Original post by tiny hobbit)
Two ways, one of which doesn't matter:

I've put in the option of energy being put into the system by an engine, cyclist etc.

My brain finds it more obvious to think of energy being taken out of the system by doing work against friction etc. This is why I have it as a minus on the left hand side. As you've realised, this is exactly the same as adding it on the right hand side.
ah, ok. would that come up in M2 or M3?
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4 years ago
#18
(Original post by Maths&physics)
is it a plus of minus. it makes sence that its a + because: total energy = (energy gained, like GPE, EPE, KE) + (energy taken by friction)???

or does it depend on direction/relative to the initial energy at the start?
Typo, I was editing my messed up LaTeX on that post that I completely missed out the sign error.
0
4 years ago
#19
(Original post by tiny hobbit)
I suggest

Mechanical energy at start (KE, Grav. PE,Elastic PE) + Work done on system (by engine, cyclist, someone pulling) - work done against friction or resistance = mechanical energy at the end
I have started to like this formula as it can be less confusing for students than the standard textbook method.

One downside with using it is that a part a) might be “find the potential energy lost by the system”. I find that students who use the formula you suggested will find that okay but then not use their answer for part b) so it will slow them down.

Also potential energy of a system of particles could be confusing because you have to choose the place for PE = 0.
0
4 years ago
#20
(Original post by Maths&physics)
ah, ok. would that come up in M2 or M3?
Without the elastic PE, it is part of the M2 syllabus. So it could also come up in M3 including elastic PE.
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