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Homework Help: Tow-truck Newton Q Pls help

  1. Oct 27, 2006 #1
    I got very confuse about this questions and really appreciate your help on this....... many thanks in advance.

    A tow truck of mass 1000kg is towing a car of mass 600kg and both are moving at a constant velocity of 60m/s. The total resistance to the motion is 4000N and half of this resistance acts on the car. What is the force exerted on the tow-truck by the car? (ans 2000N)

    Steps taken.
    Total mass = 1000 + 600 = 16000kg.
    Since it is moving at constant velocity, a = 0
    Hence Force = resistance force
    16000a = 4000
    a = 2.5 m/s2

    Mass of car = 600 kg, ==> force by car = 600 x 2.5 = 1500N
    Nett force on car = 1500 -2000 = -500 N

    Mass of tow-truck = 1000kg ==> force by truck = 1500 x 2.5 = 2500N
    Nett force on truck = 2500 - 2000 = 500N

    I think I am wrong somewhere, but got confused and not sure where am I wrong?

    Please help me..... thanks
  2. jcsd
  3. Oct 27, 2006 #2
    The question gives way more information then is needed.
    you already know the resistance of the car and you know that net force is 0 because of constant velocity.
  4. Oct 28, 2006 #3
    Yes, thats your mistake. Net force should be 0 since its at constant velocity. Remember Newton's 1st Law. Draw 2 free body force diagrams if it troubles you so much. That always help.
  5. Oct 28, 2006 #4

    am I right to say that since nett force is zero, therefore the forward force (by the truck and car) is equal to the resistance force.
    Since the resistance is 4000N and it is shared, therefore the force by the car is half of 4000N = 2000N.

    The rest of the info are not needed
  6. Oct 28, 2006 #5

    Doc Al

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    Staff: Mentor

    I think you've got it. Here's my version.

    The net force on each is zero. If you analyze the forces on the car, you only get two:
    -The resistance force, which you know is 2000 N (backward)
    -The force exerted by the tow-truck (forward)

    Since these are the only forces acting on the car, they must cancel. Thus you deduce that the force exerted by the tow-truck on the car (and, via Newton's 3rd law, the force exerted by the car on the tow-truck) must equal 2000 N.

    Just for fun, analyze the forces acting on the tow-truck. Hint: There are three forces acting. :wink:
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