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Homework Help: Bunch of questions

  1. Sep 22, 2004 #1
    I have 5 problems on my homework that I can get right. Probably because of somthing stupid but I have to get these done soon. I dont need the answers, just some hints or tips. I have included my work or a description on all of them.

    1.) A race car traveling at +44 m/s is uniformly accelerated to a velocity of +24 m/s over an 12-s interval. What is its displacement during this time?
    Vi = 44m/s
    Vf = 24m/s
    t = 12
    d = ?

    a = Vf-Vi
    a = -1.6
    d = 1/2at^2
    d = -115.2
    with correct sig figs

    2.) A car accelerates at a constant rate from 16 m/s to 23 m/s while it travels 130 m. How long does this motion take?
    Vi = 16m/s
    Vf = 23m/s
    d = 130m
    t = ?

    Dont know what formula/s to use for this one.

    3.) An airplane accelerates from a velocity of 22 m/s at the constant rate of 2.8 m/s2 over +504 m. What is its final velocity?
    Vi = 22m/s
    a = 2.8m/s^2
    d = 504m
    Vf = ?

    t = sqr((2*504)/2.8)
    t = 19
    Vf = Vi + at
    Vf = 75.2
    With correct sig figs 75

    4.) A person wearing a shoulder harness can survive a car crash if the acceleration is smaller than -300 m/s2. Assuming constant acceleration, how far must the front end of the car collapse if its crashes while going 93 km/h?
    Vi = 93km/h
    Vf = 0km/h
    d = ?

    What am I missing in this one?

    5.) A woman falls 1.6 m to the floor. How fast is she going when she hits the floor?
    Vi = 0m/s
    a = 9.8m/s^2
    d = 1.6m
    t = .57s
    Vf = ?
    Vf = Vi +at
    Vf = 5.6
  2. jcsd
  3. Sep 22, 2004 #2
    for the first one, find the average velocity and use the formula: d=vt
  4. Sep 22, 2004 #3
    for the 2nd question, use the formula: Vfinal squared= V1initial squared + 2ad
    (sorry, my subscript and superscript buttons don't work)
  5. Sep 22, 2004 #4
    sorry, screwed up the formula. it is: Vfinal squared = Vinitial squared + 2ad
  6. Sep 22, 2004 #5
    How do I get a without time? THanks for the fast responce btw.
  7. Sep 22, 2004 #6
    what question are U talking about?
  8. Sep 22, 2004 #7
    The second one, you said to use Vfinal squared = Vinitial squared + 2ad but I dont know how to get a without time. I am going of the formulas I have so there might be one.
  9. Sep 22, 2004 #8
    NOw I see what you mean...
    since d=vt, (average velocity, of course), you can just isolate t and solve it.
  10. Sep 22, 2004 #9
    You can use that equation because it does not require you to know time and works. I don't think you can use the other formulas.
  11. Sep 22, 2004 #10
  12. Sep 22, 2004 #11
    Sirus can help with the rest of the questions :(!!)
    Sorry, I'm tired and wanna go to bed... :zzz:
  13. Sep 22, 2004 #12
    np, thanks for the help.
  14. Sep 22, 2004 #13
    I got 1 and 2, now im looking for 3-5.
  15. Sep 22, 2004 #14
    #3: [tex]v_{f}^{2}=v_{i}^{2}+2ad[/tex]
    #4: You are missing acceleration, which should be set at -300 meters per sec. squared. Also convert all your values from km/h to m/s.
    #5: Use the same formula as #3.

    Most of these question use that formula, so make sure you know it for future problems.
  16. Sep 22, 2004 #15
    for number 3 im getting 5.6, is that right?
  17. Sep 22, 2004 #16
    I got one more I have to throw in
    A bus traveling at +29 km/h accelerates at a constant +3.7 m/s2 for 6.3 s. What is its final velocity in km/h?
    I was working on it here, but I cant get a correct answer. Either that or webassign is being gay.
  18. Sep 23, 2004 #17
    Try number three again.

    For this latest problem: Do you have a formula that relates velocity, accleration, and time? Rather than trying to convert meters per sec. squared to km/h, convert km/h to m/s.
  19. Sep 23, 2004 #18
    remember that acceleration is equal to the change in velocity (in the same units) over the time interval
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