Applied Mechanics and Graphic Statics
On a ladder resting on a rough ground and leaning against a smooth vertical wall, the force of friction acts

Downwards at its upper end
Perpendicular to the wall at its upper end
Zero at its upper end
Upwards at its upper end

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

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