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Kinematics/ Energy Problem

  1. Sep 20, 2015 #1
    1. The problem statement, all variables and given/known data
    A cart is initially moving at 0.5 m/s along a track. The cart comes to rest after traveling 1 m. The experiment is repeated on the same track, but now the cart is initially moving at 1 m/s. How far does the cart travel before coming to rest.



    2. Relevant equations
    W=ΔKE

    Ffd=1/2mv^2
    d is proportional to velocity squared

    3. The attempt at a solution

    0.5^2=0.25
    1^2=1
    1/0.25=4m

    The answer is 2 meters
     
  2. jcsd
  3. Sep 20, 2015 #2
    what is problem there. Do you know the exact answer. I may try. But it is not as complex as you think. it is basic. The cart displacement depends on applied force.
     
  4. Sep 20, 2015 #3
    I already posted the problem and my attempt at the solution. But I got the wrong answer.
     
  5. Sep 20, 2015 #4
    yes i got the answer
     
  6. Sep 20, 2015 #5
    It is not the energy problem
     
  7. Sep 20, 2015 #6
    it is kinematics problem. Look there is only matter time. Ok
     
  8. Sep 20, 2015 #7
    Explain..?
     
  9. Sep 20, 2015 #8
    time is equal both cases. so time is 2s
     
  10. Sep 20, 2015 #9
    That doesn't really help..?
     
  11. Sep 20, 2015 #10
    Ok, of course, you need to know the basic physics. v=x/t........0.5=1/t........t=2s...... for second case.......x=vt----------v=1*2=2m
     
  12. Sep 20, 2015 #11
    I do not believe you can just assume the times are the same.
     
  13. Sep 20, 2015 #12
    you may forget, you use the same track...........so common sense.
     
  14. Sep 20, 2015 #13
    it is not so complex. Read the book.........it is better
     
  15. Sep 20, 2015 #14
    That doesn't make sense at all... Thats like saying any two trains on the same track will stop in the same amount of time. You are completely ignoring other factors.
     
  16. Sep 20, 2015 #15
    that's why i advice you read the book Tipler physics.........it is very high level book.........so you can find the answer. In forum i cannot explain all things in a short time .......i am also busy,,,,,,,,,I just give you the right answer..........your conception you need to clear it.........I cannot do it in a short time......may be you do not have deep knowledge in physics.so it can happen.
     
  17. Sep 20, 2015 #16
    I think it is best you let someone else help on this one. You are only telling me I do not understand physics instead of solving the question correctly. I already provided you the answer and then you just told me my answer again.
     
  18. Sep 20, 2015 #17

    Doc Al

    User Avatar

    Staff: Mentor

    Why do you think the answer is 2 meters?

    I'd say that your initial answer of 4 m was correct.
     
  19. Sep 20, 2015 #18
    Thanks for your reply. I actually thought the answer was 4m based on my work above. The answer key says the answer is 2m. Therefore I clearly am doing something wrong and need help. Hence I posted on this forum...
     
  20. Sep 20, 2015 #19

    Doc Al

    User Avatar

    Staff: Mentor

    Not necessarily. The answer key can be wrong.

    The key is: what is the same between the two trails? The assumption must be that the retarding force and thus the acceleration is the same.

    Which leads to your answer of 4 m. The faster car has 4 times the energy and thus 4 times the work must be done to stop it and thus 4 times the distance.
     
  21. Sep 20, 2015 #20
    So you are saying I am correct?
     
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