Strength of Materials
Kinetic friction is the

The friction force acting when the body is in motion
Tangent of angle between normal reaction and the resultant of normal reaction and the limiting friction
The friction force acting when the body is just about to move
Ratio of limiting friction and normal reaction

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Strength of Materials
Center of percussion is

the point of C.G.
the point of application of the resultant of all the forces tending to cause a body to rotate about a certain axis
the point of metacentre
point of suspension

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