Help with M2 Conservation of Energy

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username1226161
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#1
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I'm struggling to understand why the formula given is

Gain in Kinetic Energy = 0.5mv^2 minus 0.5mu^2

yet in some questions when a particle is moving up a slope for example, the formula is switched around and apparently "v" is now the initial velocity? According to SUVAT equations, isn't V supposed to be final velocity? So confused.....
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Chlorophile
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(Original post by lightningdoritos)
I'm struggling to understand why the formula given is

Gain in Kinetic Energy = 0.5mv^2 minus 0.5mu^2

yet in some questions when a particle is moving up a slope for example, the formula is switched around and apparently "v" is now the initial velocity? According to SUVAT equations, isn't V supposed to be final velocity? So confused.....
The letters are arbitrary. The magnitude of the change in kinetic energy is always the magnitude of the difference between the start and end kinetic energy. If it's speeding up, the change is positive, if it's slowing down, it's negative.
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username1226161
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(Original post by Chlorophile)
The letters are arbitrary. The magnitude of the change in kinetic energy is always the magnitude of the difference between the start and end kinetic energy. If it's speeding up, the change is positive, if it's slowing down, it's negative.
So I always do INITIAL minus FINAL?
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Chlorophile
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(Original post by lightningdoritos)
So I always do INITIAL minus FINAL?
Final minus initial gives you the change in KE. If it's asking for the magnitude of the change, you make that positive.
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username1226161
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(Original post by Chlorophile)
Final minus initial gives you the change in KE. If it's asking for the magnitude of the change, you make that positive.
I just don't understand why for example in this question:

http://www.examsolutions.net/maths-r...tutorial-1.php

the guy does 0.5mu^2 minus 0.5mv^2 when it should be the other way round?
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Chlorophile
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(Original post by lightningdoritos)
I just don't understand why for example in this question:

http://www.examsolutions.net/maths-r...tutorial-1.php

the guy does 0.5mu^2 minus 0.5mv^2 when it should be the other way round?
I don't really want to watch an 8 minute video, when does he do this?

Don't stick rigidly to a formula. If an object is slowing down, the KE change will be negative. If it's speeding up, it'll be positive.
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