1. Not finding help here? Sign up for a free 30min 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!

With what speed does the car then strike the tree?

  1. Feb 28, 2008 #1
    1. The problem statement, all variables and given/known data
    THe driver of a car slams on the brakes when he sees a tree blocking the road. The car slows uniformly with acceleration 5.10m/s^2 for 4.40s, making straight skid marks 64.6 m long ending at the tree. With what speed does the car then strike the tree?

    2. Relevant equations
    vxf=vxi+axt
    d=(vi+vf/2)t


    3. The attempt at a solution

    a=5.10m/s^2
    t=4.40s

    - - - - -
    a=5.10m/s^2
    _________________________________
    vi=0 4.40s

    d=64.6m

    d=(vi+vf/2)t
    64.6m=(om/s+vf/2)4.40

    Then I would solve for vf, right?

    Thank you very much
     
  2. jcsd
  3. Feb 28, 2008 #2

    Kurdt

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I'm having trouble deciphering what you've done. You need to find the initial speed first, then you can find the final speed.
     
  4. Feb 28, 2008 #3
    Thank you

    Wouldn't vi be 0? If it isn't 0, then I could't determine the initial speed and the final speed if I'm not given either one, right? because I could use the equation vxf=vxi+axt, but if I don't know what vf or vi are, then I can't solve this, right?

    Is this completely wrong? if so, could you please give me a hint?

    Thank you very much
     
  5. Feb 28, 2008 #4

    Kurdt

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    what you're trying to solve for is the final velocity. If the initial velocity was zero then that wouldn't make sense since the driver wouldn't have to brake at all. What you need to do is find the initial velocity from the information given before finding the final velocity. Think of a kinematic equation involving intial velocity, acceleration, distance and time.
     
  6. Feb 28, 2008 #5
    I know that xf=xi+vxit+1/2axt^2 Does this look right?

    64.6m=0m+vxi(4.40s)+1/2(5.10m/s^2)(4.40s^2)
    vxi=3.462

    vxf=vxi+axt

    vxf=3.462m/s+5.10m/s^2(4.40s)
    =25.902m/s

    Thank you very much
     
  7. Feb 28, 2008 #6

    Kurdt

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The initial speed will not be 3.462m/s. Remember to be careful with signs.
     
  8. Feb 28, 2008 #7
    64.6m=0m+vxi(4.40s)+1/2(5.10m/s^2)(4.40s^2)

    I can't seem to find what I'm doing wrong. Xf is 64.6m and xi is 0m, right?

    Thank you
     
    Last edited: Feb 28, 2008
  9. Feb 28, 2008 #8

    Kurdt

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Well if you're slowing down the acceleration will be in the opposite direction to the direction of travel and it will therefore be negative.
     
  10. Feb 28, 2008 #9

    DaveC426913

    User Avatar
    Gold Member

    How long would a road have to go unused in order for someone to find it blocked by a tree?

    This is the dumbest physics problem evar!
     
  11. Feb 28, 2008 #10
    Would this work?

    x=64m
    a=-5.10m/s
    t=4.4s

    x=vit+1/2at^2

    64m=vi(4.4s)+1/2(-5.1m/s)(4.4s)^2

    vi=25.8m/s

    vf^2=vi^2+2ax

    vf^2=25.8m/s^2+2(-5.10m/s^2)(64m)

    =3.6m/s

    Thank you
     
  12. Feb 29, 2008 #11

    Kurdt

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    That looks ok to me.
     
  13. Feb 29, 2008 #12
    Thank you very much

    Regards
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?



Similar Discussions: With what speed does the car then strike the tree?
Loading...