Applied Mechanics and Graphic Statics
The Law of Polygon of Forces states that

If forces acting on a point can be represented in magnitude and direction by the sides of a polygon in order, the forces are in equilibrium
If forces acting on a point can be represented in magnitude and direction by the sides of a polygon taken in order, then the resultant of the forces will be represented in magnitude and direction by t
If forces acting on a point can be represented of a polygon taken in order, their sides of a polygon taken in order, their resultant will be represented in magnitude and direction by the closing side
If a polygon representing the forces acting at point in a body is closed, the forces are in equilibrium

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Applied Mechanics and Graphic Statics
‘u₁’ and ‘u₂’ are the velocities of approach of two moving bodies in the same direction and their corresponding velocities of separation are ‘v₁’ and ‘v₂’. As per Newton's law of collision of elastic bodies, the coefficient of restitution (e) is given by

e = v₂ - v₁/u₁ - u₂
e = v₁ - v₂/u₂ + u₁
e = u₂ - u₁/v₁ - v₂
e = v₁ - v₂/u₂ - u₁

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