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Homework Help: By how much does the speed change each second?

  1. Jan 30, 2010 #1
    1. The problem statement, all variables and given/known data
    A vehicle moves in a straight line with an
    acceleration of 3 km/h2.
    By how much does the speed change each
    second?
    Answer in units of km/h.


    2. Relevant equations
    Not sure


    3. The attempt at a solution
    sqrt 3 to get a base answer then divide by 3600
    I have also tried doing the problem directly. 3 divided by 36002 . Gives me a large number im unsure is correct.

    But when i don't get it right i start guessing.
     
    Last edited by a moderator: Mar 7, 2013
  2. jcsd
  3. Jan 30, 2010 #2

    tiny-tim

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    Welcome to PF!

    Hi kremit! Welcome to PF! :smile:

    (try using the X2 tag just above the Reply box :wink:)

    What equations do you know relating speed and acceleration? :smile:
     
  4. Jan 30, 2010 #3
    Re: homework help

    i have a change in speed over a change in time to equal velocity. I also have an average and instantaneous speed too. I also fixed the ^2 to the correct super script. I think that is what that is called. lol

    It's strange though. There is no other figures, that could be why im stuck.
     
    Last edited: Jan 30, 2010
  5. Jan 31, 2010 #4

    tiny-tim

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    Hi kremit! :smile:

    (just got up :zzz: …)
    No, change in distance over change in time equals velocity (∆x/∆t = v).

    ok, that doesn't help here, so …

    do you have a similar equation that includes acceleration? :smile:

    Yeah! Super! :biggrin:
     
  6. Feb 4, 2010 #5
    Re: homework help

    Why must this be so hard for me. I don't understand that.

    I looked in my book,on the web, and not making sense of what a "resultant displacement" is.

    The problem to this is simple.

    Guy passes a 241km marker, passes 172km marker, and hits a 148km marker. he then backtracks to the 172km marker. What is the resultant displacement
     
  7. Feb 5, 2010 #6

    tiny-tim

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    Hi kremit! :smile:

    (are you ok on the previous question?)

    Displacement means total change in position.

    For example, if you go half-way round a circle of radius r, then you have gone a distance (or arc-length) of πr, but your displacement is only 2r.

    And if you go all the way round a circle of radius r, then you have gone a distance (or arc-length) of 2πr, but your displacement is zero! :smile:

    (and "resultant" always simply means "overall", you take everything into account, and then calculate the displacement :wink:)

    So what is the overall displacement in this case?​
     
  8. Feb 6, 2010 #7
    Re: homework help

    A change in total position. Wonder why they couldn't say that? :redface:

    Guy passes a 241km marker, passes 172km marker, and hits a 148km marker. he then backtracks to the 172km marker

    He went the entire distance of

    93km from 241km -> 148km
    69km from 241km -> 172km
    24km from 172km -> 148km

    93km - 24km ?
    93km + 24km?

    I doubt it's that simple, but he traveled the whole distance and came back.

    I like how our lab or lecture has nothing to do with our homework. Then must rely on others or tutors to teach me equations. :uhh:
     
  9. Feb 6, 2010 #8

    tiny-tim

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    Hi kremit! :smile:

    Where did he start from? The 241 km marker? :confused:

    If so, just follow the definition … total displacement is |241 - 172|.
     
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