Strength of Materials
Center of percussion is

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 C.G.
the point of metacentre
point of suspension

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Strength of Materials
A semi-circular disc rests on a horizontal surface with its top flat surface horizontal and circular portion touching down. The coefficient of friction between semi-cricular disc and horizontal surface is i. This disc is to be pulled by a horizontal force applied at one edge and it always remains horizontal. When the disc is about to start moving, its top horizontal force will

remain horizontal
unpredictable
slant down towards direction of pull
slant up towards direction of pull

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Strength of Materials
According to principle of moments

the algebraic sum of the moments of any two forces about any point is equal to moment of theiwesultant about the same point
if a system of coplanar forces is in equilibrium, then the algebraic sum of their moments about any point in their plane is zero
if a system of coplanar forces is in equilibrium, then their algebraic sum is zero
positive and negative couples can be balanced

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