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Homework Help: I could use some basic physics help

  1. Mar 1, 2009 #1
    ive been sick a lot lately, and have missed a lot of physics, my worst class, and im doing all the homework ive missed and cant figure out these 2 problems.everything else has been done. any help, like how to do them, i dont need the answer just how to set it up and such. anything would be helpfully. im an 11th grader, so its not college leveled.

    "A 2500kg car is moving at a speed of 20m/s. the car is slowing don with a constant force of 500N
    how long does it take to stop,how far does the car tavel as it comes to a stop,make a Force vs Time"

    "A train with a mass of 20,000 Kg is moving at 3m/s and attaches 2 other trains with the same mass,already attached. the two other cars are moving with a sppeed of 1.2m/s and moving in the same direction as the first car.
    what is the speed of the three joined cars, after the collision? change of kinetic energy during collision. what sort of collision has occured?"
    Last edited: Mar 1, 2009
  2. jcsd
  3. Mar 1, 2009 #2
    or no help:(
  4. Mar 1, 2009 #3
    First question

    Mass of train = 2500Kg
    Velocity Initial = 20m/s
    Force = Mass x Accel

    500N = 2500Kg x Acceleration

    Once you find acceleration (or deceleration in this case), you'd have

    Velocity Initial = 20m/s
    Veloctiy final = 0m/s
    acceration = 500N/2500Kg

    Then you can look for distance and time...

    Second question

    Thinking of conservation of momentum...

    Momentum = mass x velocity

    (20,000Kg)(3m/s) + (40,000Kg)(1.2m/s) = (60,000Kg)(x)

    and change of kinetic energy of what during the collision?
  5. Mar 1, 2009 #4


    User Avatar

    For the first question you'll want to use Newton's second law, F = MA to find the acceleration A. Then you can use

    [tex] V = V_o + At [/tex]

    where [tex] V_o [/tex] is the initial velocity, and V, your final velocity will be, of course, zero. Make sure that your acceleration is negative.

    The how far the car goes can be found from

    [tex] X = X_o + V_o t + \frac{1}{2} A t^2 [/tex]

    You should start your second question with the conservation of momentum:

    Momentum before the collision = Momentum after the collision

    Before you have two objects traveling at different speeds and after the collision you have one object (the three coupled cars) traveling with one speed.

    After you find the velocity of the coupled cars, you can find the kinetic energy before the collision and the kinetic energy after the collision. Since these kinetic energies will be different, this is an inelastic collision.
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