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Homework Help: Forces help

  1. Sep 9, 2008 #1
    1. The problem statement, all variables and given/known data
    A car of mass 2 (metric) tonnes is traveling at 20m/s. caculate its kinetic energy (can and have done) and the force required to stop it in 10 s.


    2. Relevant equations



    3. The attempt at a solution
    I have found the KE = 4 *10^5 but i cant figure out how to calculate the force required to stop it in 10s. I think the equation for this mite be E/t=FV

    The answer is 4*10^3N
     
  2. jcsd
  3. Sep 9, 2008 #2

    danago

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    To change it speed from 20m/s to 0m/s requires a deceleration of how much? What net force is required to produce this deceleration (what law relates net acceleration of a body to the net force)?
     
  4. Sep 9, 2008 #3
    so is it F=ma
    that would give F=2000*-20
    which = 4*10^4 not 10^3
    also the force is positive but i suppose you just swap it because it is in the opposite direction to the original?? Also what then puts the 10s into the equation??
     
  5. Sep 9, 2008 #4

    danago

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    Yes use F=ma, but think about what value of 'a' you have used. a, acceleration, measures the rate at which speed is changing. If the speed changes from 20 to 0 over a period of 10 seconds, what is the acceleration?
     
  6. Sep 9, 2008 #5
    ok i think i get it now the acceleration should = 20-0/10
    which = 2
    so F= 2000*2
    =4*10^3
    thankyou so much
    This website is ausome...
     
  7. Sep 9, 2008 #6

    danago

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    No problems :smile: And yes this website is the best haha.
     
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