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Homework Help: Questions in linear motion

  1. Jun 3, 2012 #1
    http://store2.up-00.com/May12/Ueu86896.jpg [Broken]

    someone here told me that travel distance is = vs = 30 m

    now remaining is 60 - 30 = 30 m

    now my questions is ...

    to solve this Q we use which distance is travel distance or remaining distance
    Last edited by a moderator: May 6, 2017
  2. jcsd
  3. Jun 3, 2012 #2
    How far is he from the man when he hits the brakes?
  4. Jun 3, 2012 #3
    Is travel distance - total distance = 60 - 30 = 30 m
  5. Jun 3, 2012 #4
    Right, 30m.

    So that should answer your part b, and it changes your s from 60 to 30 in your calculation of acceleration.
  6. Jun 3, 2012 #5

    now for time required and acceleration we use not this distance we use travel distance which s =vt right ?
  7. Jun 3, 2012 #6
    That equation is only for uniform velocity, it can't be used when the object is accelerating.

    And the question asks for the time required to stop the car. So your answer has two parts, the time while the driver is reacting (given to be 5s) and the time it takes him to stop after hitting the brakes (which we said covers a distance of 30 meters. What other things do you know about this time period?).
  8. Jun 3, 2012 #7
    see may you don't understand me

    I mean

    v^2 = u^2 + 2as
    the answer will be - 0.66

    now for this we use s travel distance which s =vt not 60 - 30 = 30 m

    But in part B we use the distance is 60 - 30 because question wants distance cover after applying break

    correct ??
  9. Jun 3, 2012 #8
    What values are you using for your variables?
    I'm not sure what you're plugging in.
  10. Jun 3, 2012 #9
    let me give give step by step

    X ........................ 0 ..................... Z

    see now form x to 0 is distance travel when he apply brakes that we got
    s = vt = 30 m
    now from 0 to z that is remaining distance 60 - 30 = 30 m

    so we have now 3 distance total which is 60 and from starting to where he stop (X to 0 ) is 30

    and from where he stop to a man = 30 (X to Z )

    now the Q ask find acceleration (retardation )

    v^2 = u^2 + 2as
    the answer will be - 0.66

    so my question is here we must use distance form X to 0 or from 0 to Z

    so I want you tell me any of those distances used to solve time required to stop the car and uniform retardation .

    I hope that clear
  11. Jun 3, 2012 #10
    Well in which stretch (X to O or O to X) are the brakes being applied?
  12. Jun 3, 2012 #11
    X to O
  13. Jun 3, 2012 #12
    Which letter is the man and which is the initial position of the car?
  14. Jun 3, 2012 #13
    initial position of the car? is X
    man is Z

    I told you before
  15. Jun 3, 2012 #14
    So if the man is driving at the man, and after he reaches point O he starts to brake, doesn't that mean the accelerating is done from O to Z?

    The number is the same, but you need to understand what is going on in each of the two sections of this problem to understand it.
  16. Jun 3, 2012 #15
    Ok I want know some think after O is the car move Or at O the car stop fully ?
    Last edited: Jun 3, 2012
  17. Jun 4, 2012 #16
    The man doesn't hit the brakes until he gets to O, so how can he be stopped at O?
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