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Calculating Tractive Force using two methods.

  1. Aug 5, 2016 #1
    1. A car accelerates 1800m down an incline of 1 in 4 at 0.4ms^2. The car has a mass of 4,000kg and the resistance to motion is 400N


    a) The Tractive effort required by using D'Alemberts principle
    b) The Tractive effort required by using the conservation of energy

    2. Relevant equations
    a)F=ma, mgsin(theta)
    b) KE=1/2mv^2, PE=mgh

    3. The attempt at a solution

    a) calculating intertial resistance
    4,000 x 0.4 = 1600N

    calculating the angle of 1in 4

    calculating gravitational force
    4000 x 9.81 x sin(14.04)=9519.593816N

    I'm stuck after this, do I add my two values and my resistance to motion or subtract them?

    As for part b) I'm at a loss because I don't have any values for velocity to work out the change in KE nor do I have the height to work out PE. What am I missing here?

    Thanks everyone!

  2. jcsd
  3. Aug 5, 2016 #2


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    Gold Member

    whether you add or subtract depends on the direction down or up the plane of the gravity force, resistance force, inertial force, and tractive force, which must sum to 0 using D'Alemberts principle. Once you establish the direction of the first 3, the direction and magnitude of the Tractive force will follow.
    Use kinematic equations for finding V and trig for finding h. The length and slope of the incline is given.

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