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Grade 12th passMechanics

A weightless rod of length 100 cm is supported
horizontally at A and B. A weight of 50 kg is attached on the rod at C. If the support at A cannot bear a pressure more than 30 kg, the distance of C from A when A is about to fall is

Profile image of Praveen S
6 Years agoGrade 12th pass
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1 Answer

Profile image of ROSHAN MUJEEB
6 Years ago

To solve this problem, we need to analyze the forces acting on the rod and apply the principles of static equilibrium. When the rod is supported at points A and B, and a weight is hanging from point C, we can determine the distance from point A to point C just before A fails under the load. Let's break this down step by step.

Understanding the Setup

We have a rod that is 100 cm long, supported at two points, A and B. A weight of 50 kg is applied at point C. The maximum pressure the support at A can handle is equivalent to 30 kg. To find the distance from A to C at which A is about to fail, we need to use the concepts of moments and equilibrium.

Key Concepts

  • Moments: The moment about a point is the product of the force and the perpendicular distance from the point to the line of action of the force.
  • Equilibrium: For the system to be in equilibrium, the total moments about any point must equal zero.

Setting Up the Equation

Let’s denote the distance from A to C as x cm. The distance from B to C will then be (100 - x) cm. The weight of 50 kg exerts a force due to gravity, which we can calculate as:

Force due to weight (F) = mass × gravity = 50 kg × 9.81 m/s² = 490.5 N

Since A can only support a maximum force of 30 kg, the maximum force at A is:

Maximum force at A = 30 kg × 9.81 m/s² = 294.3 N

Moment Calculation

Taking moments about point A, we can write the equation based on the equilibrium condition:

Sum of moments about A = 0

The moment due to the weight at C is:

Moment at C = Weight × Distance from A = 490.5 N × x

Since A is about to fail, the reaction force at A (let's denote it as R_A) is at its maximum value of 294.3 N. The moment due to R_A about point A is zero because it acts at the pivot point.

Equilibrium Condition

We can equate the moments to find x:

490.5 N × x = 294.3 N × (100 - x)

Rearranging this gives us:

490.5x = 29430 - 294.3x

784.8x = 29430

x = 29430 / 784.8

x ≈ 37.5 cm

Final Answer

The distance of point C from point A, when A is about to fall, is approximately 37.5 cm. This means that if the weight is placed further away from A than this distance, it would exceed the support limit at A, leading to a failure of the support.