Two equal billiard balls are in contact on a billiard table. A third equal ball strikes them symmetrically and remains at rest after impact. The coefficient of restitution is

Dear Fawz,

let third ball approach the two stationary balls with speed v, let coefficient of restitution be e

from symmetry we can find out that the component of momentum normal to approaching ball is conserved

now consider parallel component: mv=2mucos30

mv=mu√3, coefficient of restitution=u/v=1/√3 

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