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Homework Help: Finding time and velocity

  1. Dec 2, 2015 #1
    1. The problem statement, all variables and given/known data
    A motorcycels accelerates at 1.6m/s^2 to reach a velocity of 20m/s of distance 50m . Determine the initial velocity and time elapsed .

    2. Relevant equations

    3. The attempt at a solution
    to find the velocity , i have 2(1.6)(50) = (20^2)- (u^2) , u = 15.5m/s
    but i have problem of finding the time ,
    why i cant use the eq of s= ut +1/2(a)(t^2) ,
    50 = 0+1/2(1.6)(t^2) , t= 8s ,
    50 = 15.5(1) +1/2(1.6)(t^2) , t= 6.56s
    the given time = 2.8s
  2. jcsd
  3. Dec 2, 2015 #2
    First equation, ut=0
    Second equation, ut=15.5(1)
    Both are wrong. Check again the formula.
  4. Dec 2, 2015 #3


    User Avatar

    Staff: Mentor

    You have chosen the correct equation but you are misapplying it. Your second version is closer, as you've correctly used the 15.5 m/s initial velocity, but for some reason you've replaced the t that multiplies it with a "1". Leave the t there. You should end up with a quadratic equation in t.

    You might also want to keep a few more decimal places in the value of u when you do additional calculations with it. Otherwise rounding errors can creep into your significant figures as you proceed. Only round values at the end for presentation.
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