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Car Colision and speed question

  1. Oct 22, 2005 #1
    Okay here is my question I have spent 6 hours on it and I can't get the answer. the Question is:
    a 920kg car is driving down the road and hits a 2300kg SUV stopped at a red light. The two cars lock together and their break lock. they then slide forward 2.8m. A police officer know that the coeficient of friction between rubber and pavement is 0.8. Whith this he can figure out the speed at which the car was going when it hit the SUV. Find the speed of the car just before the collision.

    I really hope someone can help me.
  2. jcsd
  3. Oct 22, 2005 #2


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    You are given the the masses of the cars and so can find the weight of the two cars together. That, multiplied by the coefficient of friction gives the force slowing the two cars and so the deceleration. Write the equations for the distance and speed as a function of time (which will depend on the speed of the two cars together just after the collision). Since you know the distance they moved (2.8 m) and their final speed (0 m/s) you can find their speed immediately after the collision. Use conservation of momentum to find the speed of the moving car just before the collision.
  4. Oct 22, 2005 #3
    wat equation would i use to find out the decceleration?
  5. Oct 22, 2005 #4

    You use the total normal force to find the friction force, then you find the acceleration and finally find the v0 velocity...
  6. Oct 22, 2005 #5
    my first post!

    You should draw out the force components, use rectilinear motion equations to find v_f of stationary object, and then solve for v_i of the car that caused the collision.

    For Bodies in Equilibrium The sum of the Forces is zero, viz. Newton's 1st law of motion: sum of unbalanced external forces contribute to motion of the object.

    S := Summation

    S ( F_x = 0 )

    Horizontal forces are:
    1.) Friction; F_f against motion (just throwing this bit in: Friction is always parallel to the contacting surfaces (perpendicular to the normal force) and always acts in the direction opposing motion.)

    2.) F (time derivative of momentum), which is the unbalanced force, is directed along the plane of motion.

    Time-independant equation for plane motion or rectilinear motion.
    v^2 = v_o^2 + 2*a*(s - s_o) :: s and s_o are final and initial positions of the object.

    Lastly use Conservation of Momentum: S (m_i * v_i) = S (m_i' * v_i')
    [which is always true]
    Last edited: Oct 22, 2005
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