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Homework Help: Help! With Physics problems i'm having trouble!

  1. Sep 29, 2004 #1
    1)Find the stopping distance of that same car when it is traveling up a 17.1° slope, and it locks its wheels while traveling at 34.9 m/s (78.2 mi/hr). Assume that mu_k does not depend on the speed.


    2)A mass of 4.100 kg is suspended from a 1.430 m long string. It revolves in a horizontal circle.
    The tangential speed of the mass is 2.899 m/s. Calculate the angle between the string and the vertical (in degrees).



    I'm having trouble with this two problems. The first one all i think of are the formulas : a=v^2/(2(distance), and i have to find the distance, but i dont understand :Assume ...
    In the second problem i thought i could get the angle by find the inverse cosine of the ropes lenght, but im not that sure
     
    Last edited: Sep 29, 2004
  2. jcsd
  3. Sep 29, 2004 #2

    Alkatran

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    Step 1: What is the data I have?
    v(i) = 34.9m/s
    v(f)
    mg(y) = 9.8m/s^2
    mg(parrallell) ' to slope = 9.8*sin(17.1)
    mg(perpendicular) = 9.8*cos(17.1)

    Alright, what don't we know?
    mu_k
    t
    d

    Uh oh, without those 3 we can't solve mu_k. That means we can't find the friction. You're not telling me all the data.

    2. You'll need the formula for centripedal acceleration. a = v^2/r
     
  4. Sep 29, 2004 #3
    Actually this was a two part problem the first part was:
    A 680.0 kg car travelling on a level road at 27.0 m/s (60.5 mi/hr) can stop, locking its wheels, in a distance of 61.0 m (200.1 ft). Find the size of the horizontal force which the car applies on the road while stopping.
    The answer for that one was : 4.06E+03 N
     
  5. Sep 29, 2004 #4

    Alkatran

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    Find mu_k with the first question and the second becomes possible.
     
  6. Sep 29, 2004 #5
    I used the data I have, but either the formula i'm using is incorrect or i'm doing something wrong. I tried using a=V^2/2(d) and i tried to solve for d there but i don't get a correct answer. Can you recomend any formula for this maybe a variation of the 3rd law( i assume this is a 3rd law problem).
     
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