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college physics

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A(n) 13,300 lb railroad car traveling at 7.4 ft/s couples with a stationary car of 6790 lb. The acceleration of gravity is 32 ft/s^2. What is their velocity after the collision (ft/s)?

And what impulse did the first car receive? Answer in units of lbs.

Any help would be grateful, THANKS

  • college physics -

    Use conservation of momentum. When the cars are coupled, they have the same velocity

    13,300lb*7.4 ft/s = (13,300+6790)lb*Vfinal

    It is not necessary to convert the weight unit to mass in this case. g would appear on both sides and cancel out.

    Vfinal = 7.4 ft/s*(0.662) = 4.9 ft/s

    The impulse received by the first car equals its loss of momentum:
    (13,300/32.2)slug *(4.9 - 7.6)ft/s
    = -1115 slug ft/s (or lbf-seconds)

    Mass units of slugs are needed when calculating momentum.

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