? asked in 科學數學 · 1 decade ago


You and a friend live 6miles apart and are talking on the phone. You

hear a crack of thunder and 8 seconds later your friend hears the

crack. Find an equation that gives the possible places where the lightning could have occurred. (Use miles as the uint of distance . The speed of sound is about 1100 ft/sec.)

1 Answer

  • 1 decade ago
    Favorite Answer

    This is a 3-D problem so we set locations in x,y, and z coordinates:

    Without loss of generality, your position A (-3, 0,0), your friend's location B(3,0,0), and P(x,y,z) is where the thunder occur.

    distance AB=1,100*8/5,280=88/52.8 miles is fixed, call it s

    According to the description, we are looking for the unknown point P such that PB-PA= s. Using distance formula we arrive at the collection

    of points in 3-D space:

    {P(x,y,z)| sqrt[(x-3)^2+y^2+z^2]-sqrt[(x+3)^2+y^2+z^2]= s}.

    Thus the desired equation is

    sqrt[(x-3)^2+y^2+z^2]-sqrt[(x+3)^2+y^2+z^2]= s

    If use basic algebra to simplify it we will find {P(x,y,z)} is one piece [since x less than 0] of the surface in the form x^2/a^2-y^2/b^2+z^2/c^2=1,which is classified "Hyperbola of two sheets".

    A 2-D version may help us understand P(x,y) better. It will result that {P(x,y)} is one branch of the hyperbola x^2/a^2-y^2/b^2=1, where a=s/2, c=3, and b positive such that a^2+b^2=c^2. This branch (x negative) is the projection of the answer to our problem down to xy-plane.

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