Why not use SUVAT in this situation?

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runny4
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For the paper below, in question 12d, they use energy considerations to solve the problem but why could you use SUVAT with s=139m u=57 v=? a=-9.8

In the examiners report it says Centres should note that equations of motion are for use on rectilinear motion andtherefore energy conservation should be used to answer part (d) but im not sure what this means and why SUVAT can't be applied here.


http://qualifications.pearson.com/co...e_20100128.pdf - Paper

http://qualifications.pearson.com/co...1001_Stand.pdf - Mark Scheme

http://qualifications.pearson.com/co...f_20100310.pdf - Examiner's report
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lerjj
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(Original post by runny4)
For the paper below, in question 12d, they use energy considerations to solve the problem but why could you use SUVAT with s=139m u=57 v=? a=-9.8

In the examiners report it says Centres should note that equations of motion are for use on rectilinear motion andtherefore energy conservation should be used to answer part (d) but im not sure what this means and why SUVAT can't be applied here.


http://qualifications.pearson.com/co...e_20100128.pdf - Paper

http://qualifications.pearson.com/co...1001_Stand.pdf - Mark Scheme

http://qualifications.pearson.com/co...f_20100310.pdf - Examiner's report
Whilst SUVAT will give you the correct answer, the justification is not straightforwards. The equations you'd be writing down would look like this:

 v^2 = u^2 + 2as with a = -g

But that doesn't desecribe what's going on! since SUVAT is only for cases when the motion is in a straight line, this appears to be modelling what happens if you start to travel straight up, having an initial vertical velocity equal to your current horizontal velocity. It is not clear at all whether this will give the correct speed since it is not, in fact, what happens.

In fact, the only simple justification I can think of of using SUVAT would have to use energy arguments, and by that point you may as well do the calculation correctly.
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Student403
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It actually does come out to the same calculation but as the above user said, rectilinear motion is motion in a straight line which this is not.

Our energy conservation argument neglects any of the movement around the curve at the top since it only considers height and Ek so we can use it.
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runny4
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(Original post by lerjj)
Whilst SUVAT will give you the correct answer, the justification is not straightforwards. The equations you'd be writing down would look like this:

 v^2 = u^2 + 2as with a = -g

But that doesn't desecribe what's going on! since SUVAT is only for cases when the motion is in a straight line, this appears to be modelling what happens if you start to travel straight up, having an initial vertical velocity equal to your current horizontal velocity. It is not clear at all whether this will give the correct speed since it is not, in fact, what happens.

In fact, the only simple justification I can think of of using SUVAT would have to use energy arguments, and by that point you may as well do the calculation correctly.
Thank you- that was very well explained
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runny4
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(Original post by lerjj)
Whilst SUVAT will give you the correct answer, the justification is not straightforwards. The equations you'd be writing down would look like this:

 v^2 = u^2 + 2as with a = -g

But that doesn't desecribe what's going on! since SUVAT is only for cases when the motion is in a straight line, this appears to be modelling what happens if you start to travel straight up, having an initial vertical velocity equal to your current horizontal velocity. It is not clear at all whether this will give the correct speed since it is not, in fact, what happens.

In fact, the only simple justification I can think of of using SUVAT would have to use energy arguments, and by that point you may as well do the calculation correctly.
Sorry for renewing this old thread but i was thinking why can u use SUVAT for projectile motion because it is motion in a curve not a straight line
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lerjj
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(Original post by runny4)
Sorry for renewing this old thread but i was thinking why can u use SUVAT for projectile motion because it is motion in a curve not a straight line
Because you can deal with motion in orthogonal directions independantly. I.e. what's going on in x is not affected by what's going on in y.

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(Actually, this is a little confusing, as you don't actually even need an orthogonal basis. Newton's laws work perfectly well in any affine coordinate system i.e. those where the coordinate vectors stay the same)


For a particle falling down a ramp however, the same point does not apply - if you try and resolve vertically and horizontally then the normal reaction will mess things up. If you try and resolve along the slope then your basis needs to rotate as you descend which messes up the independent motions a bit.

This isn't the best explanation, sorry. I can't think of a convincing reason why rectilinear motions are independant beyond the fact that that's how vectors work, which isn't too satisfying.
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runny4
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(Original post by lerjj)
Because you can deal with motion in orthogonal directions independantly. I.e. what's going on in x is not affected by what's going on in y.
Spoiler:
Show

(Actually, this is a little confusing, as you don't actually even need an orthogonal basis. Newton's laws work perfectly well in any affine coordinate system i.e. those where the coordinate vectors stay the same)

For a particle falling down a ramp however, the same point does not apply - if you try and resolve vertically and horizontally then the normal reaction will mess things up. If you try and resolve along the slope then your basis needs to rotate as you descend which messes up the independent motions a bit.

This isn't the best explanation, sorry. I can't think of a convincing reason why rectilinear motions are independant beyond the fact that that's how vectors work, which isn't too satisfying.
ok thanks yeah i think i get it. basically vertical motion is in a straight line and horizontal is in a straight line but these are independent of each other so u can use suvat
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