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Homework Help: Distance Traveled By A Car-Kinematics Problem

  1. Oct 29, 2009 #1
    1. The problem statement, all variables and given/known data

    a car traveling due east on a straight road and accelerates at a constant rate for 10(s) increasing its velocity from rest to 40 m/s. It then travels at a constant velocity for 5s and accelerates at a steady rate for the next 5s to a velocity of 20m/s. It travels at this velocity for 10s and then accelerates rapidly to a stop at a time of 2s. What is the total distance traveled by the car?

    All I know is, the answer NEEDS TO BE 770m. I'm trying to find the soltuion :(

    2. Relevant equations

    none /:)?

    3. The attempt at a solution

    I tried solving it first by drawing a diagram and then , paired up (40 m/s, 10s), (40m/s,5s) , (20m/s,5s), (20m/s,10s), (v=?,2s)

    then, I solved for the acceleration for each with V=Vo + at. after which, I solved for each of their distances with V(^2) = Vo(^2) + 2ad.

    for example,

    G: 40 m/s , 10s

    V = Vo+ at
    40 = 0 + a(10)
    a = 4 m/s^2

    V^2 = Vo2 + 2ad
    1600 = 0 + 2(4)d
    d = 200 m

    I did the same thing with all the others and after reaching (20 m/s,10s), I got 900. I stopped there and didn't solve further cause I thought I wouldn't get the right answer anymore even if I did solve for (v=?,2s) .

    Can someone please guide me? :( I need to find out how to solve this because it would most likely be in the exam I need to pass just days away :( It looked simple at first but I'm going crazy solving it o_O
  2. jcsd
  3. Oct 29, 2009 #2
    Do you double-check your calculation? If yes but you still have wrong answer, please post your full solution.
  4. Oct 29, 2009 #3


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    Homework Helper

    Draw the time-velocity graph.
    To find the total distance traveled by the car, find the area bounded by the graph with x-axis. It contains 2 triangles, two rectangles and one trapezium.
  5. Oct 29, 2009 #4
    how do I draw a time-velocity graph :((? I'm sorry but I really don't know how to -___-
  6. Oct 29, 2009 #5
    I did and I'm sure It's wrong :D I think my formulas is wrong
  7. Oct 29, 2009 #6
    Are 2 of your accelerations negative? If not, then I believe this is where you are making the mistake. For example, if you are going from 40 m/s to 20 m/s in 5s, this will have a negative acceleration since you are decreasing in velocity.
  8. Oct 29, 2009 #7


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    Homework Helper

    Take time on x-axis and velocity in y-axis.
    Answer to the first part is correct.
    For second part, d2 = v*t.
    For third part vi = 40 m/s and vf = 20 m/s. Do the calculation as in the first part.
    repeat the same thing for reaming parts.
    Last edited: Oct 30, 2009
  9. Oct 30, 2009 #8
    when velocity changes and decreases does that mean it has a negative acceleration? because it's slowing down right?

    I tried this and it worked :) thank you thank you!
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