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rather interesting M1 kinematics question

So here it is:
A particle is projected vertically upwards with a speed of 30ms^-1 from a point A. The point B is h metres above A. The particle moves freely under gravity and is above B for a time 2.4 s. Calculate the value of h.
(A bit weird) lol
How do I solve it? I have like 5 pages of seemingly pointless working and probably coincidentally got the answer 39 m (2 s.f)
I know I cannot have done the right working since I equated an expression of length to a time lol and despite when solving for h I know what I did was wrong.
I hate mechanics lol :tongue:
thank you!!!

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Original post by MathMeister
So here it is:
A particle is projected vertically upwards with a speed of 30ms^-1 from a point A. The point B is h metres above A. The particle moves freely under gravity and is above B for a time 2.4 s. Calculate the value of h.
(A bit weird) lol
How do I solve it? I have like 5 pages of seemingly pointless working and probably coincidentally got the answer 39 m (2 s.f)
I know I cannot have done the right working since I equated an expression of length to a time lol and despite when solving for h I know what I did was wrong.
I hate mechanics lol :tongue:
thank you!!!


h = ut+1/2 at^2

h = u(t+2.4) + 1/2 a(t+2.4)^2


Solve the quadratic for t then find h
Original post by TenOfThem
h = ut+1/2 at^2
h = u(t+2.4) + 1/2 a(t+2.4)^2
Solve the quadratic for t then find h

Surely to solve for t (quadratic) then I cannot have h (2 unknowns) therefore I need to equate the quadratic to something?
Is it (t+2.4) because the displacement after u(t+2.4) + 1/2 a(t+2.4)^2 will be h (on either side of the diagram)???
Original post by MathMeister
Surely to solve for t (quadratic) then I cannot have h (2 unknowns) therefore I need to equate the quadratic to something?
Is it (t+2.4) because the displacement after u(t+2.4) + 1/2 a(t+2.4)^2 will be h (on either side of the diagram)???


erm

h=h so you just put the RHS = to each other


and yes, the displacement is h at time=t and at time=t+2.4
Original post by MathMeister
Surely to solve for t (quadratic) then I cannot have h (2 unknowns) therefore I need to equate the quadratic to something?
Is it (t+2.4) because the displacement after u(t+2.4) + 1/2 a(t+2.4)^2 will be h (on either side of the diagram)???


Personally, I would calculate the time taken until the particle reaches its maximum height, so when v = 0. Then I would subtract 1.2 seconds from this time (the particle spends 1.2 second between B and maximum height) then work out how high it got in this time, which is h.

This is coming from someone who hasn't done mechanics in 2 and a half years though.

Let me know if you want more specific advice.
Original post by TenOfThem
h = ut+1/2 at^2

h = u(t+2.4) + 1/2 a(t+2.4)^2


Solve the quadratic for t then find h

Double the max height subtract u(t+2.4) + 1/2 a(t+2.4)^2 =h makes more sense lol
Original post by TheIrrational
...

Similar to what I done except I went full retard and said 45.91836735+4.9t^2-30t=1.2 lel (length+ an expression for length in terms of time= time) lolf
As you can see my minds language is mechanics :tongue:
(edited 9 years ago)
Original post by MathMeister
Similar to what I done except I went full retard and said 45.91836735+4.9t^3-30t=1.2 lel
As you can see my minds language is mechanics :tongue:


Wow, I never liked mechanics either. Pure is the best thing.
Original post by MathMeister
Double the max height subtract u(t+2.4) + 1/2 a(t+2.4)^2 =h makes more sense lol


I have no idea what you are planning to do "lol"

But if you have a method that you prefer then use it
(edited 9 years ago)
Original post by TheIrrational
Wow, I never liked mechanics either. Pure is the best thing.

Yeah. C3 seems 10x easier than this at the mo haha.
Original post by TenOfThem
lololololooloilololololholeololoalorlolodlolyooloulolilololololklololololelololololdlololololzlolololoalololmlolololiolololololorlololoilololgolololholololtlololoooololololololololol?lololololol-

lol yh ok lol. yh lol.
I will check to see if yur method works lol. please wait a minute :smile:
(edited 9 years ago)
Original post by TheIrrational
Personally, I would calculate the time taken until the particle reaches its maximum height, so when v = 0. Then I would subtract 1.2 seconds from this time (the particle spends 1.2 second between B and maximum height) then work out how high it got in this time, which is h.



nice method :smile:
Original post by MathMeister
lol yh ok lol. yh lol.
I will check to see if yur method works lol. please wait a minute :smile:


erm
lol
sure
it does
Original post by MathMeister
Yeah. C3 seems 10x easier than this at the mo haha.


Yep, I agree! In terms of my marks I had C3 = FP2 = M2 > C4 > M1 > M3 > M4

So in general, mechanics was worse!

In terms of this question I like TenOfThem's method. Probably the fastest possible way.
Original post by TheIrrational


In terms of this question I like TenOfThem's method. Probably the fastest possible way.


I like my method too but I like yours - I tend to "forget" the symmetry method

All Pure > Mechanics > Decision >>>>>>>>>>>>>any stats
Original post by TenOfThem
nice method :smile:


Thank you, I would have included some SUVATs in my explanation like you, but can't for the life of me remember them!

Not as efficient as your method, but it's just the only way I could break it down in my head at 10 to 12.
Original post by TenOfThem
I like my method too but I like yours - I tend to "forget" the symmetry method

All Pure > Mechanics > Decision >>>>>>>>>>>>>any stats


Couldn't agree with this more! I keep trying stats and keep realising why I hate it again and again. So I decided to become an actuary...
Original post by TheIrrational
Couldn't agree with this more! I keep trying stats and keep realising why I hate it again and again. So I decided to become an actuary...

My mind doesn't click for mechanics like it does for pure(can do c2 in <<<30 mins the correct methods<<<5 mins in my head- 5 mins just cus the sheer amount)))). I cannot really say for statistics as I've not done much. I find it rather fun anyhows hehe - i know- baffled- lol
(edited 9 years ago)
Original post by MathMeister
My mind doesn't click for mechanics like it does for pure. I cannot really say for statistics as I've not done much. I find it rather fun anyhows hehe - i know- baffled- lol


Mechanics is just practise and knowing some formulas to me. Once you can break down the problems in your head in mechanics and you know your formulas it becomes easy. I hate memorising formulas, which is one of the things I love about pure.
Original post by TheIrrational
Mechanics is just practise and knowing some formulas to me. Once you can break down the problems in your head in mechanics and you know your formulas it becomes easy. I hate memorising formulas, which is one of the things I love about pure.

???c3 trig???
I suppose people learn the why of the formulas and get familiar with the method and questions and when to use the formulas so there's no need to memorise.

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