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Homework Help: A boat moves 15 km

  1. Sep 30, 2012 #1
    1. The problem statement, all variables and given/known data
    A boat moves 15 km as it accelerates at 3.5 m/s^2 for 1.5 minutes. What is the truck's initial velocity?

    2. Relevant equations

    i'm stuck on what formula to use....

    3. The attempt at a solution[/]
    vf^2=vo^2+2ax? ....
  2. jcsd
  3. Sep 30, 2012 #2

    Simon Bridge

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    What does the truck have to do with the boat?

    If you are stuck on a formula, draw the v-t diagram.
    The boat starts at some speed u at t=0 and increases it's speed to speed v at t=90s.
    This gives you a trapezium shape with two unknown points in it.

    The slope of the line is the acceleration - which you have been given.
    The area under the graph is the distance - which you have been given.
    The rest is basic geometry.

    (The other approach is to list all the kinematic equations, and list the variables you know. The equation with all the ones you know and the one you want to find is the correct one.)
  4. Oct 1, 2012 #3
    You will have to employ your elementary equations of motion in Physics. This question is quite basic. I'm guessing the truck and the boat are the same thing and you just mixed them up.
    The SUVAT equation you used is incorrect because you are not given your final velocity so employing v^2=u^2+2as is not helpful because you're just introducing more unknown variables.

    Below is the correct method of solving this question if you don't want to sketch a graph. Although it's good to know how to do them.

    s (displacement) =15km=15000m
    a (acceleration) =3.5
    t (time) =1.5mins=90
    u (initial velocity)=?

    s = u*t+(0.5)at^2

    have a go at that, that should do it.
  5. Oct 1, 2012 #4

    Simon Bridge

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    The suvat method is focussed around the discipline of laying out the variables and choosing the equation. The bit where the numbers go into the equation is just a formality when you think about it.

    Giving someone the specific equation to use, therefore, removes the skill part of the problem as well as all the physics and turns the experience into a mindless algorithm -- all without actually solving the problem of how to choose the right equation. You may as well program a suvat app on a phone! Thus the student is unlikely to learn how and will need the same help next time.

    It is a point in pedagogy that people who have trouble with one method will benefit from learning another.

    eg. Those with trouble figuring the suvat equation to use often do better geometrically ... if they can get past the common fear of sketching graphs.
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