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Homework Help: Simple question of acceleration

  1. Feb 21, 2010 #1
    1. The problem statement, all variables and given/known data
    Ok I have been given an assignment and asked to answer questions about a lorry loaded and unloaded. the Text is as follows:
    An unloaded lorry of mass M kg accelerates from rest and reaches a velocity of 30mph in 10 seconds. It then travels at constant velocity for half a minute before the driver applies the brakes and brings the vehicle to rest over a distance of D m.
    The lorry is then loaded with 800 kg of building materials that need to be transported over a total distance of 0.6 km. After loading, the lorry accelerates over a distance of 80m to attain a velocity of 30mph again. The vehicle subsequently maintains constant velocity before decelerating to rest at a rate of R m/s2.

    M=2300KG D = 70m R = 1.5m/s^2

    2. Relevant equations
    The questions exactly
    Find for the unloaded lorry:

    i) the force required to accelerate the vehicle, and the retarding force necessary to bring it to rest;
    ii) the total time and the total distance travelled during this period.

    Find for the loaded lorry :

    i) the force required to accelerate the vehicle and the retarding force necessary to bring it
    to rest;

    ii) the total time taken during this period and the distance travelled at constant velocity.

    c) Calculate the % change in the momentum of the lorry from loaded to unloaded, whilst travelling at the specified constant velocities.

    d) Draw the ‘velocity-time’ graph of the motion of the lorry in its unloaded and loaded states, indicating clearly salient points and values.

    3. The attempt at a solution
    I tried using V^2 = U^2 +2as , V=S/t and general thinking but im 100% sure that im incorect. i got answers like 0.65 m/s^2 which seemed possible but it is half the acceleration than when the lorry was unloaded, i think half is too little if only 800KG was added. i also got stupid answers like 15.67 m/s^2

    can some one help me
    Last edited: Feb 21, 2010
  2. jcsd
  3. Feb 21, 2010 #2


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    Homework Helper

    Read your own post again and you'll see why I can't understand it at all. What information is provided about the unloaded lorry? What's the actual question? Why does the 0.6 km matter?
  4. Feb 21, 2010 #3


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    Gold Member

    I believe you left out some info regarding the unloaded lorry, like its acceleration, speed, etc. And you left out the question as well. Please re-write this problem in its entirety.
  5. Feb 21, 2010 #4
    hey i edited the original message. i think i have got the full way to B) correctly. I need B to continue though, so you can see my de lima. I usually know what im talking about when it comes to physics but this has really stumped me!! any help is a positive result :)

    Also does anyone know the value of e in the equation of electric field strength ( E = 1 / 4(pie) eo er x Q1Q2/r^2 )?
    Last edited: Feb 21, 2010
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