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# Energy Problem watch

1. 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!
2. (Original post by 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!
What is the actual question number for the PAT question?
3. (Original post by 16Characters....)
What is the actual question number for the PAT question?
Oh yes sorry - Q25
4. (Original post by PhyM23)
Oh yes sorry - Q25
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.
5. (Original post by 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.
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?)
6. (Original post by 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?)
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.
7. (Original post by 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.
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?
8. (Original post by 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?
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)
9. (Original post by 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)
Yes that is exactly what I was asking - thank you very much! It's perfectly clear now

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