Theory of Machine
The primary unbalanced force due to inertia of reciprocating parts in a reciprocating engine is given by (where m = Mass of reciprocating parts, ω = Angular speed of crank, r = Radius of crank, θ = Angle of inclination of crank with the line of stroke, and n = Ratio of the length of connecting rod to radius of crank)

m.ω².r sinθ
m.ω².r (cos 2θ/n)
m.ω².r cosθ
m.ω².r (sin 2θ/n)

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Theory of Machine
In order to give a complete secondary balance of a multi-cylinder inline engine,

Both (A) and (B)
The algebraic sum of the secondary forces must be equal to zero
The algebraic sum of the couples about any point in the plane of the secondary forces must be equal to zero
None of these

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