Class 9 > Unit # 04: Turning Effect of Forces > Centre of Gravity

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Turning Effect of Forces - Centre of Mass & Gravity

Define Centre of mass or centre of gravity. Describe method to find centre of mass of irregular objects.

Centre of Mass or Centre of Gravity

"A body behaves as if its whole mass is concentrated at one point, called its centre of mass or centre of gravity, even though gravity attracts every part of it."

Figure: Centre of gravity of regular and irregular shaped objects.

Examples for Regular Objects

• Uniform Rod: The center of gravity is located at its geometric midpoint.
• Uniform Square or Rectangular Sheet: The center of gravity is at the exact point of intersection of its diagonals.
• Solid or Hollow Sphere: The center of gravity is at the geometric center of the sphere.

Centre of Gravity of an Irregular Shaped Thin Lamina (Plumb Line Method)

To find the center of gravity of an irregularly shaped object (thin lamina), follow these experimental steps:

1. Step 1: Make three small holes near the edges of the lamina, placing them as far apart from each other as possible.
2. Step 2: Suspend the lamina freely from one of the holes on a retort stand using a pin or nail.
3. Step 3: Hang a plumb line (a string with a heavy weight attached) from the same pin in front of the lamina.
4. Step 4: Wait until the plumb line comes to a complete rest, then carefully trace the line of the string onto the lamina with a pencil.
5. Step 5: Repeat steps 2 through 4 for the second and third holes. The point of intersection of all three drawn lines gives the precise position of the centre of gravity of the irregular lamina.

Figure: Centre of gravity of irregular shaped thin lamina.

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