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Homework Help: Force Problem with car

  1. Feb 3, 2008 #1
    1. The problem statement, all variables and given/known data
    A man moves two passenger cars by clamping his teeth don a bit that was attached to the cars with a rope and then leaning backward while pressing his teeth against the railway ties. The car together weighed 700 kN. Assume that he pulled with a constant force which was 2.5 times his body weight, at an upward angle of 30 degrees from the horizontal. His mass was 80 kg, and he moved the cars 1.0m. Neglecting any retarding force from the wheel rotation, find the speed of the cars at the end of the pull.

    d = 1.0m
    theta = 30 degrees
    force total = 2.5 x man's body weight
    cars' weight = 700kN
    man's mass = 80kg

    2. Relevant equations
    Just need help to get started. Thanks in advance.

    3. The attempt at a solution
    Don't understand how to find force only given distance and no time.
  2. jcsd
  3. Feb 3, 2008 #2
    I am going to assume that since it is not mentioned, that this whole system is frictionless. Set up your coordinate system so that the origin is at the point of where the rope attaches to the cars. Do not tilt the coordinate system. Draw a free body diagram. You will have a normal force in the positive y direction and mg in the negative y direction. You will also have the force exerted on the cars (tension of the cable) by the man pulling with his teeth. You need to solve for the x and y components of this tension using trig. The hypotanuse of the right triangle will be the mass of the man times 2.5. All you really need is the force in the x direction since you are solving for velocity in the x direction. Once you have this force, you will be able to find the acceleration in the x direction using F=ma. Plug that into a 2D kinematics equation not involving time and you will have your velocity.
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