A 300-kg truck moving rightward with a speed of 5 km/hr collides head on with a 1000kg car moving leftward with a speed of 100 km/hr. The two vehicles stick together and move with the same velocity after the collision determine the post – collision speed of the car and truck


  1. Answer:

    The final velocity of the vehicles will be -75km per hour (leftwards).


    Let us call m_1, v_1 mass and velocity of the truck , m_2, v_2 mass and velocity of the car, and v_f the final velocity of the vehicles.

    The law of conservation of momentum states that

    m_1v_1-m_2v_2= v_f(m_1+m_2)

    solving for v_f we get:

    v_f = \dfrac{m_1v_1-m_2v_2}{m_1+m_2}

    putting in numerical values

    m_1 =300 kg \\v_1 =5km/hr = 1.39m/s\\m_2 = 1000kg\\v_2 = 100km/hr = 27.78m/s

    v_f = \dfrac{300*(1.39)-1000*(27.78)}{300+1000}

    \boxed{v_f = -21.0m/s } (the negative sign indicates that the vehicles are moving in the leftward direction)

    or -75.8 km per hour.

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