Physics homework help

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Sandrey546
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#1
Report Thread starter 3 years ago
#1
Just started AS and already confused about physical quantities. I can do most questions but I’m stuck on this one asking to prove it’s homogeneous.

E = 1/2pr^5f^2

I got L.H.S to be kgm^2s^-2 but I got the R.H.S to be kg^2m^2s^-2

I don’t know how to do it with one less mass pls help, thanks in advance.
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Zinc~
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#2
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#2
Hello I've done a level physics, what equation is this? Is E energy?
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Sandrey546
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#3
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#3
To be honest I’ve never seen this equation before for all I know it could be made up, the question does not say what any of them are so I am guessing that E = energy p = density (this symbol was used in a previous page for density) r for radius and f for force
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Zinc~
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#4
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#4
I think this question is incorrect,
you got the energy spot on E, Work done = force times distance. F =ma, kgms^-2 times m = kgm^2s^-2.

RHS ~ density = mass/volume, kgm^-3, the p is rho which donates density in phyics
Radius = m^5
Force = kg^2m^2s^-4

grouping ~ i got kg^3m^4s^-4 so my conclusion is that it is not homogeneous.
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Sandrey546
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#5
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#5
OH ok. Just one more thing, it asked to do the same thing with the ke = 1/2mv^2 equation. I could do this question if it asked me for the base units of Ke, but how would I turn Ke into base units besides using that equeation because is the only one I can think of.
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Zinc~
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#6
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#6
Yeah KE is just energy, all energy has the same SI units. You could use anything Work done = force times distance. or E=mgh. The trick to homogeneous style question is to try and think of the most simple formulae to use. so for energy I would always go for Work Done = Force times Distance. Work done is in J which is the same unit as energy so Work Done can be thought of as energy.
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Sandrey546
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#7
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#7
Oh, that really clears things up now. Thanks so much!
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Zinc~
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#8
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#8
Yup these questions will soon seem simple try to memorise the SI units. Just remember to focus on the symbols and powers, the coefficient in front is irrelevant (eg 1/2 mv^2 just ignore the 1/2) I remember thats what confused me at first. Apart from that its just grouping like terms and adding powers. Also try to appreciate the reason behind this, you're basically seeing if an equation could work, for an equation to be correct the units on either side must be equal!
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