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# M2 - GPE, KE, Friction formula?? Watch

1. Ive seen the formula Work done against friction = Loss in GPE– Gain in KE
used in a lot of mark schemes. How does this work and what questions do you use it for? Ive never seen it in the books.
2. (Original post by fpmaniac)
Ive seen the formula Work done against friction = Loss in GPE– Gain in KE
used in a lot of mark schemes. How does this work and what questions do you use it for? Ive never seen it in the books.
It's just conservation of energy. The total energy has to remain constant. So if both GPE and KE change, that is, their sum is not the same, the energy must have gone into work being done against friction since total energy is conserved.
3. (Original post by Zacken)
It's just conservation of energy. The total energy has to remain constant. So if both GPE and KE change, that is, their sum is not the same, the energy must have gone into work being done against friction since total energy is conserved.
So when theres friction you would use the above formula but when there isnt you would use GAIN IN KE = LOSS IN PE..
4. (Original post by fpmaniac)
So when theres friction you would use the above formula but when there isnt you would use GAIN IN KE = LOSS IN PE..
It depends on the question.

E.g. If you're moving up a slope then you'll be gaining GPE and losing KE. If there's no friction then Gain in GPE = Loss in KE.

As Zacken has said, try to understand the energy changes instead of relying on a formula.

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