# AS mechanics

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#1
I'm confused with SUVAT equations when trying to work out t, i get different answers when using different equations to solve for t, could anyone help,
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1 year ago
#2

(Original post by beekydog)
I'm confused with SUVAT equations when trying to work out t, i get different answers when using different equations to solve for t, could anyone help,
0
1 year ago
#3
(Original post by beekydog)
I'm confused with SUVAT equations when trying to work out t, i get different answers when using different equations to solve for t, could anyone help,
If you're using different equations, you're probably substituting the wrong variable into the ones that aren't the correct one.

The best thing to do is to list all the variables you are given, and the variable that the question is asking you for. e.g.

a=2, s=5, u=1, v=?

Then pick the formula that has all four of the variables in it and plug your numbers in.
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#4
s=-1.5 u=22.4 v=0 a=-9.8 t=?
when I use s=(u+v/2)t and s=ut+0.5at2 i get different answers
(Original post by mqb2766)
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1 year ago
#5
s= t*(u+v)/2
are your brackets correct? Distance is average velocity * time.

What was the original question?
(Original post by beekydog)
s=-1.5 u=22.4 v=0 a=-9.8 t=?
when I use s=(u+v/2)t and s=ut+0.5at2 i get different answers
0
1 year ago
#6
(Original post by beekydog)
s=-1.5 u=22.4 v=0 a=-9.8 t=?
when I use s=(u+v/2)t and s=ut+0.5at2 i get different answers
s=(u+v/2)t this is not valid, you should re-derive the equations again. it is s = (u+v)t/2 ; classic mistake made at a level, which is to try to memorise every equation. Work on understanding the theory and then just derive the equations yourself, having a solid understanding of what is happening is far more useful
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#7
ok, thank you! why isn't it valid?
(Original post by mnot)
s=(u+v/2)t this is not valid, you should re-derive the equations again. it is s = (u+v)t/2 ; classic mistake made at a level, which is to try to memorise every equation. Work on understanding the theory and then just derive the equations yourself, having a solid understanding of what is happening is far more useful
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1 year ago
#8
Because its wrong. distance is average velocity*time.
That is not average velocity, which is (u+v)/2
(Original post by beekydog)
ok, thank you! why isn't it valid?
1
1 year ago
#9
(Original post by beekydog)
ok, thank you! why isn't it valid?
its incorrect, the physics (and hence math) does not follow this rule, you cant just make up equations for convenience. As stated previously blindly trying to memorise every equation is not the best way to solve problems. I recommend learning the key classical physics principles in mechanics and derive the equations yourself. You will learn this much more effectively
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#10
I haven't learnt the equation, I'm using it straight out of my textbook and that's how it appears in the book

(Original post by mnot)
its incorrect, the physics (and hence math) does not follow this rule, you cant just make up equations for convenience. As stated previously blindly trying to memorise every equation is not the best way to solve problems. I recommend learning the key classical physics principles in mechanics and derive the equations yourself. You will learn this much more effectively
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1 year ago
#11
(Original post by beekydog)
I haven't learnt the equation, I'm using it straight out of my textbook and that's how it appears in the book
i doubt that's the equation in the book are you sure its not
s = (u+v)t/2

And once again you've proved my point, if you have just copied random equations from a book without any understanding and have made a mistake copying & re-writing
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1 year ago
#12
and many other places on the web. Just search for
deriving suvat
and it will be 1/2 hr well spent. Some may do it slightly differently, but its important to understand what the terms mean.
(Original post by beekydog)
I haven't learnt the equation, I'm using it straight out of my textbook and that's how it appears in the book
Last edited by mqb2766; 1 year ago
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1 year ago
#13
(Original post by mnot)
s=(u+v/2)t this is not valid, you should re-derive the equations again. it is s = (u+v)t/2 ; classic mistake made at a level, which is to try to memorise every equation. Work on understanding the theory and then just derive the equations yourself, having a solid understanding of what is happening is far more useful
that is inappropriate advice for average students. if you are a genius with a deep understanding of physics then by all means derive the equations in the exam room. normal people should memorise 3 suvat formulae.
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#14
I am sure that it says (u+v both over 2) multiplied by t, (sorry the way I typed it out earlier may have caused confusion aha)
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1 year ago
#15
(Original post by beekydog)
I am sure that it says (u+v both over 2) multiplied by t, (sorry the way I typed it out earlier may have caused confusion aha)
Yes, see post 5.
Do you still get a different answer? If so, can you post the numbers.
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1 year ago
#16
(Original post by the bear)
that is inappropriate advice for average students. if you are a genius with a deep understanding of physics then by all means derive the equations in the exam room. normal people should memorise 3 suvat formulae.
im not on about in the exam, im saying spending an hour or so now, to go and learn what is actually happening. Once hes done that the small mistakes will be easily avoided. Instead hes made a mistake copying a formula down, and if u blindly just copy formulas you leave yourself vulnerable to these kind of mistakes (and once you've derived it once or twice its very quick to do)
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#17
Thanks I'll watch it now
(Original post by mqb2766)
and many other places on the web. Just search for
deriving suvat
and it will be 1/2 hr well spent. Some may do it slightly differently, but its important to understand what the terms mean.
T
1
#18
Yeah I definitely don't have a good understanding of physics haha
(Original post by the bear)
that is inappropriate advice for average students. if you are a genius with a deep understanding of physics then by all means derive the equations in the exam room. normal people should memorise 3 suvat formulae.
0
1 year ago
#19
(Original post by mnot)
s=(u+v/2)t this is not valid, you should re-derive the equations again. it is s = (u+v)t/2 ; classic mistake made at a level, which is to try to memorise every equation. Work on understanding the theory and then just derive the equations yourself, having a solid understanding of what is happening is far more useful
(u+v)t/2 = (u+v/2)t
they are the same thing?
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1 year ago
#20
(Original post by Gent2324)
(u+v)t/2 = (u+v/2)t
they are the same thing?
no these are not equal: (u+v)t/2 =/= (u+v/2)t

EDIT: unless u=0
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