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Homework Help: Car Jack - need help

  1. Nov 26, 2007 #1
    1. The problem statement, all variables and given/known data


    I am trying to lift a trailer with a car jack. The trailer weighs about 2,000 lbs. The trailer has a "trailer jack" unit which is attached to the trailer. Now, I believe we have two forces so far: 1) The force of the weight of the trailer pushing down on the ground and 2) the force of the ground pushing up against the trailer.

    Here's the big question:

    If I use a car jack to exert a force upward onto the "trailer jack", is there an additional force imposed upon the trailer jack?

    My friends said that there isn't an additional force imposed on the trailer jack (which is attached to the trailer and supports the 2,000 lbs of the trailer).

    I tend to think that there are 3 forces: 1) and 2) as stated above, and 3) the additional force put out from the car jack onto the trailer jack. *Am I correct or incorrect?

    The reason why I think there is a 3rd force (or an additional force added to the ground's force pushing up) is because I was concerned that the trailer jack (which is attached to the trailer) has a limit of how much force it can handle...otherwise it may break, or bend out of shape. (The trailer jack can is designed to handle up to 2,000 lbs, I believe.) It is completely attached to the trailer and pretty much acts as one entire unit of the trailer.

    Is there an additional force or stress put on the trailer jack? Or are there only two forces that we are dealing with (an equal and opposite force)? Is the force from the car jack a non-factor in this case?

    Thanks everyone for your time. Need help!

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Nov 26, 2007 #2

    Shooting Star

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    Homework Helper

    If the trailer is completely off the ground and resting on the jack, then the force acting on the jack from above is equal to the weight of the trailer only. There is no additional force. The jack also exerts an upward force on the trailer which is equal in magnitude and holds the trailer up.

    When you are lifting the trailer, that is, when the trailer is in upward motion, then there is an extra downward force on the jack.
  4. Nov 26, 2007 #3
    Thanks, Shooting Star! That makes sense - it's equal an equal and opposite force - the force of the jack pushing up, and the force of the trailer pushing down on the jack.

    Since the trailer jack can handle the weight of the trailer and being lifted up by 2 feet (by a car jack), the trailer jack won't break as it's designed to handle 2,000 lbs, and be connected onto a trailer hitch that is about 2 feet off the ground. That was my initial worry - a worry that I shouldn't need to be concerned about.
  5. Jan 30, 2008 #4
    Hello guys about car jacks,
    I'm redesigning a Y-Jack for my D&M class. Drawing up the free-body diagrams of the jack in its lowest and highest positions it is possible to obtain the two forces acting on the jack(weight of car and normal force from ground). When the jack is split,we are supposed to be able to obtain horizontal forces at the pivots.This is where i'm stuck. I keep on getting horizontal forces as zero but there is a force which causes the threaded bar to elongate(tension).
    I need to obtain the horizontal force in the threaded bar.
    To see a Y- Jack go to www.freepatentsonline.com and sign-up.

  6. Feb 2, 2008 #5

    Shooting Star

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    Hi lildav,

    You can post this afresh in the General Physics section with a diagram. Then you may get a lot of help. This is not the right section.
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