Pressure Profile

There is a 10 kg 10 \text{ kg} box of length, height, and width ( L = 2 m , H = 0.5 m , W = 1 m ) (L = 2\text{ m}, H = 0.5\text{ m}, W = 1\text{ m}) sitting on a rough surface.

A 20 N 20\text{ N} rightward horizontal force is applied at the middle of the left side of the box at a distance of H 2 \frac{H}{2} from the bottom of the box. There is sufficient friction that the box remains stationary.

Let the variable x x represent the horizontal position (in meters) relative to the left side of the box. With the horizontal force applied, the upward pressure ( \big( in N/m 2 ) \text{N/m}^{2}\big) associated with the normal reaction varies according to the equation P ( x ) = α x + β P(x) = \alpha x + \beta .

Determine β α \frac{\beta}{\alpha} , to 2 decimal places.

Details and Assumptions:

  • There is a uniform downward gravitational acceleration of 10 m/s 2 10\text{ m/s}^{2} .
  • Assume that the box is solid and has uniform mass density.
  • Linear pressure variation is an assumption made by the problem author. See here for further discussion.


The answer is 5.67.

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1 solution

Rohit Gupta
Jan 5, 2017


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 N = \int_0^L {(\alpha x + \beta )Wdx} M g = W ( α L 2 2 + β L ) α L + 2 β = 2 M g W L = 100 . . . ( 1 ) \begin{gathered} Mg = W(\alpha \frac{{{L^2}}}{2} + \beta L) \\ \alpha L + 2\beta = \frac{{2Mg}}{{WL}} = 100 & ...(1) \\ \end{gathered}


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 H 2 + M g L 2 W ( α L 3 3 + β L 2 2 ) = F H 2 + M g L 2 2 α L + 3 β = 3 W L 2 ( F H + M g L ) = 157.5 . . . ( 2 ) \begin{gathered} {\tau _N} = {\tau _F} + {\tau _{Mg}} \\ \int_0^L {(\alpha x + \beta )Wxdx = F\frac{H}{2} + Mg\frac{L}{2}} \\ W(\alpha \frac{{{L^3}}}{3} + \beta \frac{{{L^2}}}{2}) = F\frac{H}{2} + Mg\frac{L}{2} \\ 2\alpha L + 3\beta = \frac{3}{{W{L^2}}}(FH + MgL) = 157.5 & ...(2) \\ \end{gathered}

Solving equations (1) and (2)

β = 42.5 α = 7.5 β α = 5.67 \begin{gathered} \beta = 42.5 \\ \alpha = 7.5 \\ \frac{\beta }{\alpha } = 5.67 \\ \end{gathered}

If the height varies, then why does Mg exert a torque at distance L 2 \frac{L}{2} ?

Shaun Leong - 4 years, 5 months ago

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For uniform objects, Mg acts at center of the objects which is at a distance L/2 from the side AB

Rohit Gupta - 4 years, 5 months ago

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Sorry I thought the diagram showed fluid.

Shaun Leong - 4 years, 5 months ago

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