Kinematics (projectile) - why is my solution wrong?

  • #1
Femme_physics
Gold Member
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Homework Statement



http://img29.imageshack.us/img29/4773/biker101.jpg [Broken]

A biker's track is planned so when a biker jumps of a hill slanted at a 30 degrees angle and a height of 1.4m. During a race it's been determined that the biker depicted in the drawing stays in the air 1.65 seconds. Calculate the speed of the biker as he leaves point A.

The Attempt at a Solution



http://img815.imageshack.us/img815/2316/vovovo.jpg [Broken]

Apparently this is the wrong figure. And oh yea-- I forgot to add meter per second.
 
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Answers and Replies

  • #2
38
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well i like to work back from

y=yi+viyt-(gt2)/2

try that out noting the initial and final displacements from the diagram.

okay i just derived the equation you used. the reason its not working is due to this situation being asymmetrical (initial and final displacements not the same), whereas your equation only works for symmetrical situations.

try to work from the formula i have given, as it takes less assumptions then your one, just a suggestion
 
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  • #3
Femme_physics
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okay i just derived the equation you used. the reason its not working is due to this situation being asymmetrical (initial and final displacements not the same), whereas your equation only works for symmetrical situations.

Thanks a bunch! I'll keep working on that :)
 
  • #4
38
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also note that the velocity you attain from solving my equation is only the initial VERTICAL component. therefore you have to use another formula to find the total initial velocity

ill leave that up to you ;)
 
  • #6
I like Serena
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I believed then that I used a formula that gives me V0 directly, not just its y component :)

Nice! :smile:

Just not neat. :P
 
  • #7
Femme_physics
Gold Member
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lol, I noticed I ran out of paper as I was plugging in the figures in that formula so I crossed it out. What do you expect when you don't have a decent scanner to work with?!?

Though tbh this is easier, just writing it like that and taking a webcam pic. Tons faster, too. And I noticed that using a thick black marker it's pretty clear as well. :)
 
  • #8
I like Serena
Homework Helper
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lol, I noticed I ran out of paper as I was plugging in the figures in that formula so I crossed it out. What do you expect when you don't have a decent scanner to work with?!?

Well, there's less space now for bolting bugs! :)

Though tbh this is easier, just writing it like that and taking a webcam pic. Tons faster, too. And I noticed that using a thick black marker it's pretty clear as well. :)

Yes, the thick black marker is very readable.
It's much better than what I would've expected from a webcam.

And the size in the posts is good too: not too small and not too big.

It's also nice to see a glimpse of you every now and then! :smile:
 
  • #9
38
0
nice you got it

ill say once again that, in my expierience all projectile motion questions at least at my level,
can be solved by the equation i gave you and

v=vi+at (where you once again split into vertical and horizontal components)
 
  • #10
Femme_physics
Gold Member
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I'll keep it in mind. Thanks Lackos :)

And thanks ILS for your careful analysis of...everything :-)
 

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