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Question with acceleration vector

  1. Feb 21, 2015 #1
    1. The problem statement, all variables and given/known data
    http://imgur.com/LravIr3

    2. Relevant equations


    3. The attempt at a solution
    We know A is wrong, because a object going in a circle has acceleration (i'm not sure why that is, maybe someone can explain)

    B is wrong because if a car in reverse is slowing down, it technically has positive acceleration, right?

    D is wrong because refer to my explanation for B.

    C and E, I have no idea what that formula is but V x T is distance but I have no idea what the formulas are implying. Anyone can shed some light on what C and E means?
     
  2. jcsd
  3. Feb 21, 2015 #2

    Suraj M

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    Gold Member

    The formula ##v(T + Δ T) ## refers to the velocity of the object at time## (T +Δ T)##.
    Now can you decide if C and E are right or wrong?
     
  4. Feb 22, 2015 #3
    how does velocity multiplying the time give you velocity again?
     
  5. Feb 22, 2015 #4

    Suraj M

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    Its not multiplication, It's a way of representing the velocity at a particular time, as the velocity varies with time!
     
  6. Feb 22, 2015 #5
    can you make up a word problem that uses v(T+ΔT)?
     
  7. Feb 22, 2015 #6

    Suraj M

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    Okay instead of a word problem with ##V(T+ΔT)## try this.
    Let ##~~ V(t) = ƒ(t)## and ##ƒ(t) = u(0) + at ##
    here i used ## u(0)## itts actually ##u(t=0)## we often miss out the ##t=##
    so in your question it should have been- ## v(t_2 = T+ΔT)## and ##v(t_1 = T)##
    we often omit the t= to make things easier,
    Its represented like this because velocity is not a constant and is a function of time, skips the steps you'd have to involve to define ##v_1 ~ and~ v_2##
    its just like writing ##V(initial) and V(final)##
    see this
     
    Last edited: Feb 22, 2015
  8. Feb 22, 2015 #7

    NascentOxygen

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    Staff: Mentor

    answer (D) seems true
    EDIT: (D) is not always true
     
    Last edited: Feb 22, 2015
  9. Feb 22, 2015 #8

    NascentOxygen

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    Where the acceleration of a body is known to be constant, we can determine that acceleration by making two measurements of its velocity some time apart, then using the formula;

    a = Δv / Δt

    = ( v(T+ΔT) - v(T) ) / ΔT
     
  10. Feb 22, 2015 #9
    if I'm in a car, and i'm in reverse while increasing my speed. I'm accelerating while going backwards so my acceleration is negative. I then slow down a bit while reversing but I'm still moving backwards, my acceleration is now positive even though i'm still moving backwards. It is positive because I slowed down in reverse. How can D still be true?
     
  11. Feb 22, 2015 #10

    NascentOxygen

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    Staff: Mentor

    You are right. I was not correct because in that case (D) is not true. I'll amend my earlier post. Thanks for the correction.
     
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