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Posted by on Saturday, March 3, 2012 at 12:14pm.

In a game of pool, the cue ball strikes another ball initially at rest. After the collision, the cue ball moves at 4.15 m/s along a line making an angle of -25° (i.e. below the x-axis) with its original direction of motion (the x-axis), and the second ball has a speed of 2.846 m/s.
Find the angle between the direction of motion of the second ball and the original direction of motion of the cue ball. Assume the masses are equal.

Find the original speed of the cue ball.


Is kinetic energy conserved?

  • physics - , Saturday, March 3, 2012 at 5:17pm

    According to the law of conservation of linear momentum
    vector(p)= vector(p1)+ vector(p2),
    Let vector( p) is directed along +x-axis. Then vector (p1) is
    below x-axis and vector(p2) is above it.
    Projections on axes are
    x: v = v1•cosα +v2•cosβ
    y: 0 = - v1•sin α +v2•sinβ
    sinβ = v1•sin α/v2 = 0.616.
    β = arcsin 0.616 = 38 degr.
    cos β = 0.788.
    v = 4.15•0.906+2.846•0.788=6 m/s.
    At elastic collision the kinetic energy is conserved.

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