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Newtons third law of motion

  1. Mar 10, 2010 #1
    1. The problem statement, all variables and given/known data
    A train consisting of two cars pulled by a locomotive experiences an acceleration of 0.33m/s^2 [fwd]. Friction is negligible. Each car has a mass of 3.1 * 10^4 KG.

    a. ) Determine the force exerted by the first car on the second car.
    b. ) Determine the force exerted by the locomotive on the first car.

    2. Relevant equations
    Applying newtons third law of motion which states:

    for every action force, there is a simultaneous force equal in magnitude, but opposite in direction

    3. The attempt at a solution

    I drew a FBD of the situation and solved for part a) by simply applying newtons second law equation [ F= mA ]

    I solved for F and got 1.0 * 10^4 N.

    However I have trouble solving for part b.)
    The answer is supposed to be 2.0 * 10^4 N, but I can't get it.

    Would appreciate any help/advice!
    Many thanks in advance.
  2. jcsd
  3. Mar 10, 2010 #2
    Since your answer for part a) is the force of the first car pulling on the second, now you can apply Newton's third law.
  4. Mar 10, 2010 #3
    Never mind. I've figured out the solution. The force of the locomotive on the first car involves the masses of the 1st and 2nd cars. Therefore I add the two masses together to get the total mass however I keep acceleration the same...

    Using newton's second law equation I determine the force.

    F = mA
    F = (62000)(0.33)
    Therefore F = 2.0 * 10^4 N [fwd]
    Last edited: Mar 10, 2010
  5. Mar 10, 2010 #4
    Ok. Since car 1 pulls car 2 with such a force you got in part a), car 2 must pull car 1 with an equal and oppose force according to Newton's third law. From there you can calculate the force exerted on car one by the locomotive. Maybe drawing a free body diagram would help.
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