# Energy Problem

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On the third from last part on the 2010 PAT -

http://www.physicsandmathstutor.com/pat/solutions-2010/

only the vertical kinetic energy has been used. However on Q6 part a) in this M2 paper -

http://qualifications.pearson.com/co...-June-2014.pdf

the speed gained by using Pythagoras with the horizontal and vertical components of the velocity is used when calculating the kinetic energy.

Why do you only take into account the vertical velocity in the first paper, but don't in the second?

Thanks!

http://www.physicsandmathstutor.com/pat/solutions-2010/

only the vertical kinetic energy has been used. However on Q6 part a) in this M2 paper -

http://qualifications.pearson.com/co...-June-2014.pdf

the speed gained by using Pythagoras with the horizontal and vertical components of the velocity is used when calculating the kinetic energy.

Why do you only take into account the vertical velocity in the first paper, but don't in the second?

Thanks!

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#2

(Original post by

On the third from last part on the 2010 PAT -

http://www.physicsandmathstutor.com/pat/solutions-2010/

only the vertical kinetic energy has been used. However on Q6 part a) in this M2 paper -

http://qualifications.pearson.com/co...-June-2014.pdf

the speed gained by using Pythagoras with the horizontal and vertical components of the velocity is used when calculating the kinetic energy.

Why do you only take into account the vertical velocity in the first paper, but don't in the second?

Thanks!

**PhyM23**)On the third from last part on the 2010 PAT -

http://www.physicsandmathstutor.com/pat/solutions-2010/

only the vertical kinetic energy has been used. However on Q6 part a) in this M2 paper -

http://qualifications.pearson.com/co...-June-2014.pdf

the speed gained by using Pythagoras with the horizontal and vertical components of the velocity is used when calculating the kinetic energy.

Why do you only take into account the vertical velocity in the first paper, but don't in the second?

Thanks!

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(Original post by

What is the actual question number for the PAT question?

**16Characters....**)What is the actual question number for the PAT question?

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#4

(Original post by

Oh yes sorry - Q25

**PhyM23**)Oh yes sorry - Q25

For the PAT question (assuming you mean "Using your calculation of the projectie's initial kinetic energy, again calculate the maximum altitude of the projectile" however you are trying to find when the vertical velocity is 0. This will be when the vertical kinetic energy is 0, and it will have all been converted into GPE.

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(Original post by

By the looks of it, for the M2 question you are working out how much KE overall has been converted into GPE and hence working out what height it must have gained.

For the PAT question (assuming you mean "Using your calculation of the projectie's initial kinetic energy, again calculate the maximum altitude of the projectile" however you are trying to find when the vertical velocity is 0. This will be when the vertical kinetic energy is 0, and it will have all been converted into GPE.

**16Characters....**)By the looks of it, for the M2 question you are working out how much KE overall has been converted into GPE and hence working out what height it must have gained.

For the PAT question (assuming you mean "Using your calculation of the projectie's initial kinetic energy, again calculate the maximum altitude of the projectile" however you are trying to find when the vertical velocity is 0. This will be when the vertical kinetic energy is 0, and it will have all been converted into GPE.

I think I'm starting to get it. So in the PAT question, when the particle is at maximum altitude, it still has the same amount of horizontal KE but all of the initial vertical KE is lost. But in the M2 question, the particle is coming to a stop on the platform, so the horizontal KE is also lost at this point, so you use the overall KE? (Or is this totally wrong?)

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#6

(Original post by

Haha yeah that's the right question!

I think I'm starting to get it. So in the PAT question, when the particle is at maximum altitude, it still has the same amount of horizontal KE but all of the initial vertical KE is lost. But in the M2 question, the particle is coming to a stop on the platform, so the horizontal KE is also lost at this point, so you use the overall KE? (Or is this totally wrong?)

**PhyM23**)Haha yeah that's the right question!

I think I'm starting to get it. So in the PAT question, when the particle is at maximum altitude, it still has the same amount of horizontal KE but all of the initial vertical KE is lost. But in the M2 question, the particle is coming to a stop on the platform, so the horizontal KE is also lost at this point, so you use the overall KE? (Or is this totally wrong?)

For M2 not quite; there is no need to overcomplicate this one by considering components.

The whole point of both questions is that it involves equating KE lost with the GPE gained. Not equating the initial KE with the GPE gained (it just so happens in the PAT question that the initial vertical KE was all converted into GPE so the vertical KE lost was the same as the initial vertical KE). So for the M2 question you would write

Initial (total) KE = Final (total) KE + GPE gained

Where the "Final (total) KE" is the KE possessed by the ball immediately before impact.

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(Original post by

Correct for PAT.

For M2 not quite; there is no need to overcomplicate this one by considering components.

The whole point of both questions is that it involves equating KE lost with the GPE gained. Not equating the initial KE with the GPE gained (it just so happens in the PAT question that the initial vertical KE was all converted into GPE so the vertical KE lost was the same as the initial vertical KE). So for the M2 question you would write

Initial (total) KE = Final (total) KE + GPE gained

Where the "Final (total) KE" is the KE possessed by the ball immediately before impact.

**16Characters....**)Correct for PAT.

For M2 not quite; there is no need to overcomplicate this one by considering components.

The whole point of both questions is that it involves equating KE lost with the GPE gained. Not equating the initial KE with the GPE gained (it just so happens in the PAT question that the initial vertical KE was all converted into GPE so the vertical KE lost was the same as the initial vertical KE). So for the M2 question you would write

Initial (total) KE = Final (total) KE + GPE gained

Where the "Final (total) KE" is the KE possessed by the ball immediately before impact.

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#8

(Original post by

So if I did Pythagoras on the PAT question would it still work out the same as it would if I just considered the vertical component of the velocity?

**PhyM23**)So if I did Pythagoras on the PAT question would it still work out the same as it would if I just considered the vertical component of the velocity?

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(Original post by

If you mean could you use the "overall KE" approach for the PAT question then yes you can. (though seeing as it asks for a particular method I wouldn't)

**16Characters....**)If you mean could you use the "overall KE" approach for the PAT question then yes you can. (though seeing as it asks for a particular method I wouldn't)

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