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lodonnell243
(Mar 14, 2009 at 14:29)
A real physicist would take g to be 9.81, therefore your theory is incorrect. Think of the people who have read this and are now going to get hurt... Tut tut 
my god i thort i'd escaped this once i got home. altho wile i'm here, you should include an air friction coefficient in ur answer and the friction coefficient created by the tyres on the dirt. and 9.8 is a rookie mistake and one more thing just to be annoying. always specify units of angles (degrees or radians) altho i realise it has to be degrees in this case, so i'l let u off 
lecture over hav a nice day kids!
and one more thing just to be annoying. always specify units of angles (degrees or radians) altho i realise it has to be degrees in this case, so i'l let u off lecture over hav a nice day kids!
if you was going to do that you would have to include the energy lost with suspention forks ? would you have to use inertia
this is very very very picky
but you'd could take direction into:
if an object is fired at x ms ^(-1)
and fired north, east south and west
it ill travel further when going west
but you'd could take direction into:
if an object is fired at x ms ^(-1)
and fired north, east south and west
it ill travel further when going west
evry one keeps sayin, take into consideration the rebound and preload of the forks for interia, i dont think that you can get an accurate measure ment of a forks, preload and rebound, just beacue each for is different, and no doubt each run the rider takes may be rougher or smoother then the next jump, so it might be differnt measurements each time. but props dude.
but the rebound and the preload influence, as do the air resistance, the temperature, the height, the tires pressure, the mass center, rider position (with variates during the jump).....
But in general cool and useles stuff!!!!
But in general cool and useles stuff!!!!
A real physicist would take g to be 9.81, therefore your theory is incorrect. Think of the people who have read this and are now going to get hurt... Tut tut
Re: that would only make cm's of diffence so he's right
Re: that would only make cm's of diffence
so he's right
dude, weight does not affect simple projectile motion; read up on your physics.
and if you want to include wind (which is a force as a function of velocity) it is too hard to integrate nicely
but when you're only going maybe 20 kph it doesnt do a great deal
and if you want to include wind (which is a force as a function of velocity) it is too hard to integrate nicely
but when you're only going maybe 20 kph it doesnt do a great deal
D-Monster:
You are not getting the principles right here. It is the PRINCIPLE he has not used 9.81 as a physicist that I am getting at Doing a physics A-level, I am being picky on the point I would lose marks for that rather than I am being right twat. 
You are not getting the principles right here. It is the PRINCIPLE he has not used 9.81 as a physicist that I am getting at
Doing a physics A-level, I am being picky on the point I would lose marks for that rather than I am being right twat.
oh dear lol serious mate u have too much time, just do the jump and if you fail, u try again thats my theory lol
Everybody shut up. I'm failing geometry and you guys are talking about this stuff like you talk about women. My brain is deteriorating from it all. Plus the best and easiest way to know how fast to go is to just guess it and hit it.
im passing geometry and im a year ahead of my grade 
at least im not one of the nerds 2 years ahead in trig/alg2 who might understand this shit
at least im not one of the nerds 2 years ahead in trig/alg2 who might understand this shit
just worked it out if you are in the air for 1 second you will of traveled 81.9 miles (130.3 km) therefore if you are in the air for 2.5 seconds and the jump is facing west realisticly you will be jumping 204.75 miles (329.5 km) + the length of the jump. think about that one then!!
Lol, this is what my friend just said who got an offer from cambridge to read Maths: "OMG lol! I had no idea there were so many nerds on pinkbike. I'm pretty sure though that there's an x missing three lines from the bottom (I think it should be v^2sin(theta)cos(theta)x rather than just v^2sin(theta)cos(theta)) and so basically Angus Murray has completely f*cked this up and is going to be the reason that loads of dirtjumpers hurt themselves."
My other "immensely cool" friend who's got an engineering offer from Cambridge has confirmed the above and also says:
Alichapple: You are wrong it makes no difference if you use degrees or radians
Spongosaurus: You are also categorically wrong: "if an object is fired at x ms ^(-1)
and fired north, east south and west
it will travel further when going west" no, simply no. That doesn't happen.
Angusmurray: "he is impressed with your physics but declares that your formula is messy!"
Alichapple: You are wrong it makes no difference if you use degrees or radians
Spongosaurus: You are also categorically wrong: "if an object is fired at x ms ^(-1)
and fired north, east south and west
it will travel further when going west" no, simply no. That doesn't happen.
Angusmurray: "he is impressed with your physics but declares that your formula is messy!"
well you can tell your friend to shove his diploma up his ass because that is a basic fact and if he doesnt know that then good luck with his diploma
actually, just put an x next to the sin(theta)cos(theta) on the bottom of the fraction under the square root
and its all good
im sorry if anyone got hurt
and its all good
im sorry if anyone got hurt
so the official equation is as follows::::::::
v=sqrt((-4.9*L^2)/((y2-y1)*(cos(theta))^2-cos(theta)*sin(theta)*L))
v=sqrt((-4.9*L^2)/((y2-y1)*(cos(theta))^2-cos(theta)*sin(theta)*L))
All math aside, I just take a few run-ins and hit jumps. No calculations, just balls and a tiny bit of skill. LOL @ your math and the argument that ensued.
except you're not taking into account the suspension of the bike (regardless of how small the fork of a dirt jumper is) which changes the heights/distance the rider goes significantly.
true, but still, most dj forks only have 100mm of travel so i wouldnt call the changes 'significant'.
the equation is based on the motion of a particle, and neglects factors like wind resistance, rotation, the movements of the rider, etc...
but it does give a pretty good indication of how fast you need to go.
the equation is based on the motion of a particle, and neglects factors like wind resistance, rotation, the movements of the rider, etc...
but it does give a pretty good indication of how fast you need to go.
YOU are not taking into account that suspension is really JUST A SPRING and any energy input into them is put back out, minus friction... Conservation of Energy.
i took higher physics and i hated it
so i dropped out half way through now when i look at my friends jotters its like
wow thts gay as heck im glad im out of there
so i dropped out half way through now when i look at my friends jotters its like
wow thts gay as heck im glad im out of there
does this mean i need a spedometer? i'll need to lock out my suspension, ride stiff armed and still legged. and i'll need my school supplies with me at all times. i'ld rather just huck
all you need is a tape measure, inclinometer, speedometer and calculator. i dont' see what the big deal is.
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Nice one for trying. Looks pretty good to me. Although you have not taken into consideration the loss of momentum through air resistance and friction as you leave the ramp. There are a number of factors you have chosen to ignor, so your formula is not exact. But as mathematical models go.... good work.
im working on a new model: one which incorporates wind resistance, rolling resistance, maybe fork compression, etc
shouldnt be too hard...
shouldnt be too hard...
Oh my word - this is like an endless circle, it just gets more and more complicated.
then you're probably going to have to take into consideration all sorts of quantum wotzits
then you're probably going to have to take into consideration all sorts of quantum wotzits
If you could get that done id be impressed. My maths unfortunately wouldnt be up to that right now lol. Give it a go and see how it goes.
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Why's everyone talking about "taking into account rolling resistance"? It doesn't affect the formula at all because the velocity term is the v right at the lip of the jump. Any effects of rolling resistance would already have appeared, and are thus incorporated into the velocity term.
ahhhhhhhhh!!! im only a freshmen i cant figure it out, so does this mean i cant get good untill i can do math like that??
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and make sure you have a speedo on your bike....wont be able to know how fast you are going otherwise
If you are getting bad grades just think of that Haha or just show your teacher that and see if she understands it
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yeah, the solving part of it but i'm talking about the actual formulation of the equation which is physics. sorry if that wasn't clear :/
this has been discussed so many times
it only matters how fast you are going when you hit the jump...
it only matters how fast you are going when you hit the jump...
the formula in hitting the jumps basically measures on how big are your balls....in dirt there's no physics thing...
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you would have to take into account rotational energy or inertia because otherwise you could not go forwards on a jump with a vertical takeoff. Also you would have to have an imaginary velocity since you have the root of a negative number in there either that or the length of the gap would have to be complex (just try putting some numbers in and you'll see). also if you were to take the case where the take off and landing are the same height and the takeoff was vertical you would end up dividing by 0 since (y2-y1) would go to 0 as well as sin(theta)cos(theta) which also makes it all go strange.
when you plug the numbers in you get the root of a positive number, whenever the difference between the heights (y2-y1) is less than the tangent of the theta. Also, theta has to be between 0 and 90 degrees. As you say, when theta equals 90 degrees, everything f*cks up, but thats because this particle sized bike rider would just go up and come straight back down, there is no rotational energy involved in this equation. Most jumps, even if they are extremely steep, don't go past 90 degrees, they might be at about 85 or something, but 90 is just silly.
So actually, for all theta between 0 and 90, and as long as the difference in the height is less than a meter (which is a reasonable assumption for most dirt jumps), you will not get a complex velocity.
so yeh
take that
So actually, for all theta between 0 and 90, and as long as the difference in the height is less than a meter (which is a reasonable assumption for most dirt jumps), you will not get a complex velocity.
so yeh
take that
I just stuck some numbers in (l=2.5, y2=2, y1=1.5 and theta=75) and it came out with 6.256983481i which is an imaginary number!?!?!?!
well what you should have gotten was 11.9 m/s, because i just did the exact same thing -- but thats using the wrong equation
look up a bit and you see i made a mistake, so using your values....
v=7.2 m/s
both of these are real, i dunno what you did to get complex speed
look up a bit and you see i made a mistake, so using your values....
v=7.2 m/s
both of these are real, i dunno what you did to get complex speed
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How about the old formula
IF IN DOUBT, HIT IT FLAT OUT?
Usually works for me, plus i dont need a calculator and waste 10mins measuring and working out what i know the end product will tell me - PEDAL!!
IF IN DOUBT, HIT IT FLAT OUT?
Usually works for me, plus i dont need a calculator and waste 10mins measuring and working out what i know the end product will tell me - PEDAL!!
I'll be honest here. I flunked in maths in high school. Infact I have always been failing in maths ever since 4th grade.
So I have no clue about this stuff over here.. i just know there are some trignometry formulas here.. but what they prove.. i dont know dude.
But i know if there are 9 casettes at the rear wheel... and 2 chaingrings infront that = 18 speeds!
So I have no clue about this stuff over here.. i just know there are some trignometry formulas here.. but what they prove.. i dont know dude.
But i know if there are 9 casettes at the rear wheel... and 2 chaingrings infront that = 18 speeds!
dude, i do trajectory questions like this all the time in physics! never thought to use one like this, physics is everywhere!
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very few ppl ride of jumps without pulling up so any figure you get out of this is goning to be way offf the mark.
stephen muarry put it best when he said "prettyshady ... you just got to pull back and yank"
stephen muarry put it best when he said "prettyshady ... you just got to pull back and yank"
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Sydney, Australia
8262 views | 137 comments | 459 faves
Date: 2009-03-14
Trail: arbitrary
Riders: imaginary
Description: Got bored doing physics study so I decided to spice things up a bit... hope it is useful for everyone!
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