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!

A car is traveling at a steady 74 km/h in a 50 km/h zone...

  1. Jun 3, 2016 #1
    1. The problem statement, all variables and given/known data
    acceleration =8.0m/s
    constant= 74km/h
    displacement 50 km/h?

    2. Relevant equations
    How much time elapses before the motorcycle is moving as fast as the car?
    How far is the motorcycle from the car when it reaches this speed?

    3. The attempt at a solution
    Is there a formula where i can plug my givens in?
     
  2. jcsd
  3. Jun 3, 2016 #2

    RUber

    User Avatar
    Homework Helper

    It doesn't look like your full question was posted. What is the initial velocity of the motorcycle?
     
  4. Jun 3, 2016 #3
    A car is traveling at a steady
    74 km/h in a 50 km/h zone. A police motorcycle takes off at the instant the car passes it, accelerating at a steady 8.0 m/s2 .
    How much time elapses before the motorcycle is moving as fast as the car?
    How far is the motorcycle from the car when it reaches this speed?
    I got the first question by doing v= 74*(5/18) which gave me 20.5s
    then I looked in my book and got the formula vf=vi +A(T)
    vf=20.5
    vi=0
    a=8.0
    so i set 20.5=8.0t and solved for T giving me 2.569
     
  5. Jun 3, 2016 #4
    the Second question is what i am confused on
     
  6. Jun 3, 2016 #5

    RUber

    User Avatar
    Homework Helper

    So, it takes the motorcycle 2.569 seconds to accelerate to 20.5m/s.
    How far does the motorcycle travel in 2.569 seconds?
    You can use the fact that distance travelled is the integral of velocity:
    ##s(T) = \int_0^T v(t) dt##
    and velocity for constant acceleration, as you already showed, is equal to acceleration times time.
    ##v(t) = at.##
     
  7. Jun 3, 2016 #6

    cnh1995

    User Avatar
    Homework Helper

    You can keep all the speeds in kmph. Are you familiar with the SUVAT equations?
    Edit: I read the acceleration as 8kmph/s by mistake which is actually 8m/s2. So you need to convert km into m.
     
  8. Jun 3, 2016 #7
     
  9. Jun 3, 2016 #8

    cnh1995

    User Avatar
    Homework Helper

    For the first part, from SUVAT, you can use v=u+at, which you've already done.
    For the second part, you can compute the displacemets of both the vehicles (using SUVAT) and then take the difference between them.
     
    Last edited: Jun 3, 2016
  10. Jun 3, 2016 #9

    RUber

    User Avatar
    Homework Helper

    In general, an object starting from rest with constant acceleration will cover a distance of:
    ##d = \frac12 a T^2 ## over the time from t= 0 to t= T.
    If you have an initial velocity, this is changed to
    ##d = v_0 T + \frac12 a T^2 ## over the time from t= 0 to t= T.
    I imagine the SUVAT equations should look similar.
     
  11. Jun 3, 2016 #10
    I got 26m and it was correct!! thank you so much!! :)
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted