There is a box of length, height, and width sitting on a rough surface.
A rightward horizontal force is applied at the middle of the left side of the box at a distance of from the bottom of the box. There is sufficient friction that the box remains stationary.
Let the variable represent the horizontal position (in meters) relative to the left side of the box. With the horizontal force applied, the upward pressure in associated with the normal reaction varies according to the equation .
Determine , to 2 decimal places.
Details and Assumptions:
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For the force equilibrium in vertical direction, the normal reaction by ground (N) should balance weight of the object (Mg).
The normal reaction force is distributed on the complete base. Let us assume a strip of width x at a distance x from the left face as shown. The force acting on the strip will be pressure at this strip times the area of the strip and the total normal reaction can be calculated by integrating this force from 0 to L.
N = ∫ 0 L ( α x + β ) W d x M g = W ( α 2 L 2 + β L ) α L + 2 β = W L 2 M g = 1 0 0 . . . ( 1 )
For the rotatory equilibrium, the torque of all forces about the side AB should also equal 0.
τ N = τ F + τ M g ∫ 0 L ( α x + β ) W x d x = F 2 H + M g 2 L W ( α 3 L 3 + β 2 L 2 ) = F 2 H + M g 2 L 2 α L + 3 β = W L 2 3 ( F H + M g L ) = 1 5 7 . 5 . . . ( 2 )
Solving equations (1) and (2)
β = 4 2 . 5 α = 7 . 5 α β = 5 . 6 7