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Math Help - Velocity of a projectile word problem

  1. #1
    Newbie
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    Apr 2009
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    Cool Velocity of a projectile word problem

    Heres a word problem i'm having difficulty with.

    Velocity of a projectile. The velocity v of a projectile is a linear function of time t that it is in theair. A ball is thrown downward from the top of a tall building. It's velocity is 42 ft per sec after 1 sec and 74 ft per sec after 2 sec. Write v as a linear function of t. What is the velocity when
    t = 3.5 seconds?

    this problem comes from the linear equation chapter.
    i have this to work with
    1. v = distance / time
    2. t = 3.5 sec
    3. m = y2 - y1 / x2 - x1
    4. point slope form: y-y1 = m( x-x1 )
    5. slope intercept: y = mx+b

    but what i have to work with I still dont know how to come up with the soulution can someone please come up w/ the solution. This seems like a good problem to learn for a future physics class that i'll be required to take.
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  2. #2
    Junior Member
    Joined
    Jan 2009
    Posts
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    Hmm. Let's see. Any linear function is in the form of f(x)=ax+b, with a non-zero a. Your exercise gave us two pieces of information about the velocity under time (let's call it v(t)), namely:

    v(1) = 42 and
    v(2) = 74

    So, replacing x by the appropriate values:

    42 = a(1) + b and
    74 = a(2) + b

    Two unknowns, two equations, solve the system and you get the values of a and b, giving you a model that describes the velocity of the object. Then use it to find the velocity at t = 3.5 s
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