# Rotational doubt (also momentum)

#### iitjee10

If a ball collides elastically with a stick at a point other than its centre of mass, then can we use coefficient of restitution is one or do we have to use initial kinetic energy = final kinetic energy??

If we can't use "coefficient of restitution is one" then why???

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#### jtbell

Mentor
According to the definition of an elastic collision, the total kinetic energy is conserved.

#### iitjee10

so u r telling that we cant use coefficient of restitution as zero in this case??

#### clem

so u r telling that we cant use coefficient of restitution as zero in this case??
You must mean one instead of zero.
The COF will not be one if either object starts to rotate.

#### iitjee10

Yeah i meant 1
then what will be COR??

#### Doc Al

Mentor
Why not forget about COR and just apply conservation of linear momentum, angular momentum, and kinetic energy.

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