Describe an experiment to demonstrate that there is no resultant moment on an object in equilibrium

1.5.2 Turning Effect of Forces

Objective

Demonstrate that an object in equilibrium experiences no resultant moment (turning effect). ⚖️

Key Concepts

  • Moment (Torque): The turning effect of a force about a point.

    Formula: \$M = r \times F\$, where \$r\$ is the position vector from the pivot to the point of application of the force, and \$F\$ is the force vector. The magnitude is \$M = rF\sin\theta\$.

  • Equilibrium: An object is in equilibrium when the sum of all forces and the sum of all moments about any point are zero.

  • Clockwise vs Counter‑Clockwise: Choose a sign convention (e.g., clockwise positive, counter‑clockwise negative) when adding moments.

Analogy: The Seesaw

Imagine a seesaw (a simple lever) balanced on a fulcrum. If two children of equal weight sit at equal distances from the centre, the seesaw stays level. The upward force from the ground at the fulcrum balances the downward forces of the children, and the moments they create cancel each other out. This everyday example shows that when moments are equal and opposite, there is no net turning effect. 🎠

Experiment Setup

Materials:

  • 1 ruler (30 cm) or a long wooden board

  • 1 pencil or a small cylindrical object to act as a fulcrum

  • 4 identical small weights (e.g., 50 g each)

  • Ruler or measuring tape

Safety Note: Keep the experiment area clear of sharp objects. Handle weights carefully. 🛠️

Procedure (Step‑by‑Step)

  1. Place the pencil on a stable surface and position the ruler horizontally on top of it so that the pencil acts as the fulcrum.

  2. Mark two points on the ruler at equal distances from the fulcrum (e.g., 10 cm on each side).

  3. Place one weight at each marked point.

  4. Observe the ruler. It should remain level, indicating equilibrium.

  5. Repeat the experiment with different distances (e.g., 8 cm and 12 cm) and adjust the weights so that the moments balance (e.g., 30 g at 12 cm and 20 g at 8 cm).

  6. Record all distances and weights in the table below.

Data Table

SideDistance from Fulcrum (cm)Weight (g)Moment (g·cm)
Left1050500
Right1050500
Left1230360
Right820160

Analysis

For each side, calculate the moment: \$M = r \times F\$.

If the sum of moments about the fulcrum is zero, the ruler is in equilibrium.

In the first set of data, \$M{\text{left}} = 500\$ g·cm and \$M{\text{right}} = 500\$ g·cm, so \$\sum M = 0\$.

In the second set, \$M{\text{left}} = 360\$ g·cm and \$M{\text{right}} = 160\$ g·cm, giving \$\sum M = 200\$ g·cm.

To restore equilibrium, adjust the weights or distances until the moments balance. This demonstrates that no resultant moment exists when the moments cancel. 🔧

Conclusion

The experiment confirms that an object in equilibrium experiences no net turning effect. When the moments produced by all forces about a pivot are equal and opposite, the sum of moments is zero, and the object remains level. This principle is fundamental to understanding levers, balances, and many mechanical systems. 🏗️

Exam Tips

  • Always choose a convenient pivot point for calculating moments.

  • Use a consistent sign convention (clockwise positive, counter‑clockwise negative).

  • Remember that the moment is \$M = rF\sin\theta\$; for forces perpendicular to the lever, \$\sin\theta = 1\$.

  • When given distances and forces, check that \$\sum M = 0\$ for equilibrium.

  • In diagram questions, label all forces, distances, and indicate the direction of each moment.