1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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


    User Avatar
    Science Advisor
    Homework Helper

    Step 1: What is the data I have?
    v(i) = 34.9m/s
    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?

    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


    User Avatar
    Science Advisor
    Homework Helper

    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).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook