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Dynamics, please help me start this.

  1. Feb 24, 2009 #1
    Dynamics Question

    1. The problem statement, all variables and given/known data

    1. The problem statement, all variables and given/known data
    A flat-bed truck with a frictionless ramp accelerates forward.

    (That 35 is the angle, and a 5kg block rests on the hypotenuse)

    / |
    --- / l
    - - /35deg l
    - ---------------

    The acceleration of the truck is suck that a 5.0kg block is stationary (relative to the ramp). If the rolling coefficient of friction is (mew=0.10) Find the force the engine is exerting on the 12000kg truck.

    2. Relevant equations
    F=ma Ff=mewFn Fg=mg

    3. The attempt at a solution
    Well...frankly, I really don't know how to start this question. How do I know where to start first? I'm wondering if I need acceleration in this or not, but I got a=0.98 m/s2 assuming that the truck's accel and the box's are the same.

    Ok, so I reattempted it finding Fparallel as 40.138N assuming Fparallel is = to Fn
    I found Ff as 4.0138N, and looked for Fperpendicular which = to 28.105N
    I subracted Ff from Fperpendicular to find Fnet which I got as 24.09.
    I took the Fnet and divided by the mass of the crate which is 5kg on the ramp which got me 4.82m/s2

    Assuming that the acceleration of the box, and the acceleration of the truck to be the same,
    I did Fnet=ma which is Fnet=12000 x 4.82 = 56840N

    It's a pretty big number, and I'm wondering if anyone can help me confirm if I did it right.
    Last edited: Feb 24, 2009
  2. jcsd
  3. Feb 24, 2009 #2

    Tom Mattson

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    Staff Emeritus
    Science Advisor
    Gold Member

    Re: Dynamics Question

    You always start this sort of problem by drawing free body diagrams and applying Newton's second law to them. Look at the examples in your book and notes. You should see this.

    It's true that the truck and the box have the same acceleration, but that common acceleration is not 0.98 m/s2. How did you get this?
  4. Feb 25, 2009 #3
    For the acceleration, I got that wrong so I redid it, and I ended up with a new acceleration of 5.26m/s2
  5. Apr 25, 2010 #4
    hi, i was wondering if you figured out the final answer to this question?
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