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Homework Help: Car crash problem

  1. Nov 10, 2008 #1
    1. The problem statement, all variables and given/known data
    A 1015 kg automobile hits a tree at 32 m/s, coming to a complete stop in 0.03 s. What force was applied to the car by the tree? (Take the forward direction of the car as positive).

    2. Relevant equations
    i dont know what would be relevant.
    f (cos theta) d

    3. The attempt at a solution
    the answer from the teacher is 1082667 J but i dont know how youre supposed to find that. i think im missing some equation. i looked through everything cant really find one that has to do with mass velocity and time for a force. and i have no idea how to make up equations. well i probably could but it wouldnt be right.

    thank you!
  2. jcsd
  3. Nov 10, 2008 #2
    No...the formula is

    [tex] F =ma =F \frac{ \Delta v}{\Delta t} = 1015 \left( \frac{32-0}{0.03} \right) [/tex]

    get it?
  4. Nov 10, 2008 #3
    oh yeah i meant that

    but how would you use that in the problem? because the mass is given. dont you use that? well you must have to because the answer is 1082667 J and i have no idea how to get that.
  5. Nov 10, 2008 #4
    Sorry, there should have been a m, let me try it agian:

    [tex] F =ma =m \frac{ \Delta v}{\Delta t} = 1015 \left( \frac{32-0}{0.03} \right) [/tex]

    Do you get it now?
  6. Nov 10, 2008 #5
    yes thank you so much!
    i never learned that equation until now. i knew a=delta v/ delta t but thats it. i didnt know it was force.

    thanks again
  7. Nov 10, 2008 #6
    That's because it isn't force only multiplied with a mass it becomes a force.
  8. Nov 10, 2008 #7
    yeah i definitely feel stupid but now i get it!
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