Theme: Mechanics (Moments & Static Equilibrium)
Educational Context: The Principle of Moments
For a rigid body to be in rotational equilibrium, the sum of clockwise moments (torques) about any pivot point must equal the sum of counter-clockwise moments. A moment is calculated as Force $\times$ Perpendicular Distance to the pivot ($M = F \times d$). Crucially, for a "uniform beam," its own weight acts exactly at its geometric center (center of mass).
Step-by-Step Analysis:
1. Establish the Framework: - Total length = 3.0 m. Let the left end be $x = 0$. The right end is $x = 3.0$.
- The string (pivot) is at $x = \mathbf{1.0 \text{ m}}$.
2. Locate the Forces and Calculate Distances from Pivot:
- Load 1 (300 N): Placed 0.5 m from the left end ($x = 0.5$). Distance to pivot = $1.0 - 0.5 = \mathbf{0.5 \text{ m}}$. This creates a Counter-Clockwise (CCW) moment.
- Beam Weight (100 N): Acts at the center ($x = 1.5$). Distance to pivot = $1.5 - 1.0 = \mathbf{0.5 \text{ m}}$. This creates a Clockwise (CW) moment.
- Load 2 (80 N): Must be placed on the right side to balance the massive 300 N weight on the left. Let its distance from the pivot be $d$. It creates a Clockwise (CW) moment.
3. Set up the Equilibrium Equation:
- $\sum M_{\text{CCW}} = \sum M_{\text{CW}}$
- $(300 \text{ N} \times 0.5 \text{ m}) = (100 \text{ N} \times 0.5 \text{ m}) + (80 \text{ N} \times d)$
- $150 = 50 + 80d$
- $100 = 80d implies d = 100/80 = \mathbf{1.25 \text{ m}}$.
4. Translate distance to the final answer:
- The 80 N weight is 1.25 m to the right of the pivot.
- Its position on the beam is $x = 1.0 (\text{pivot}) + 1.25 = \mathbf{2.25 \text{ m}}$ from the left end.
- The question asks: "How far from the other end (the right end, $x=3.0$)".
- Distance from right end = $3.0 - 2.25 = \mathbf{0.75 \text{ m}}$.
Common Pitfalls & Exam Strategy:
- Forgetting the Beam's Weight: This is the #1 mistake. If you don't account for the 100N weight of the beam acting at its center, your equation will be $150 = 80d implies d = 1.875$, leading to Option D (1.125m) or a confused guess. Always draw the weight vector at the center of a uniform object.
- Reading the Final Question: Solving for $d$ gives 1.25m (Option C), and solving for position gives 2.25m (Option B). You must read exactly which distance they want as the final answer.
Answer:→A) 0.75m