Finding Centre of Ruler

No matter where you place your fingers under a meter stick, if you slide them together, they will always meet at the stick’s center of gravity. This surprising result shows how friction and weight distribution interact to ensure the balance point is always found.

1. Place a meter stick across your two index fingers. Start with one finger at each end of the stick.
2. Slowly slide your fingers toward each other. They will meet at the stick’s center of gravity, where it balances.
3. Repeat the experiment, but this time start with one finger at the end of the stick and the other finger halfway between the end and the center.
4. Ask the class to predict where the fingers will meet. Most will expect them to meet at a point closer to the middle finger’s starting position.
5. Slide your fingers together. They still meet at the center of gravity of the stick, even though you started unevenly.

Finding the center of mass of a ruler using the sliding fingers technique - ISLE Physics:

📄 Big Stick Balance Easy Science Experiment - Science Fun for Everyone: https://www.sciencefun.org/kidszone/experiments/big-stick-balance-easy-science-experiment/

• Place coins or weights on one end of the stick to shift the balance point, then test different starting positions again.
• Try with a broom handle or ruler of different lengths.

• Use lightweight sticks or rulers to avoid injury if dropped.
• Secure loose coins or weights so they do not fall off.
• Perform the activity over a table to prevent objects from dropping onto feet.

• Why do the fingers always meet at the balance point, no matter where they start? (The heavier side presses harder, creating more friction, so it slips less while the lighter side slides more. This shifting continues until balance is reached.)
• What happens when extra weight is added to one side? (The balance point shifts toward the heavier side, but your fingers still find it automatically.)
• Why is this demonstration surprising? (It feels like your starting position should affect the result, but friction forces make the outcome independent of where you begin.)
• How does this relate to everyday life? (Balancing loads on trucks, carrying trays, or tightrope walking all rely on understanding center of gravity.)