Physics Demo Number: 062

Approximate Run Time: 5 min

Center of Mass of an Irregularly-Shaped Board

Demo Description

Secure an irregularly shaped rigid plane body by successive single points, with its plane vertical. Let gravity help determine its center of mass using intersecting plumb lines.

Scientific Principles

• Center of Mass

• Torques and Rotational Equilibrium

Equipment

• Irregularly-shaped brown board

• Thin rod

• Tripod Stand

• Plumb Line

Equipment Location

• The board is on [E-1-4].

• The thin rod and Plumb Line are in Kit (062) on [E-2-4].

• The tripod stand that has its top two attachment fixtures already configured to accept the rod (as in the first photo) is at the start of the [H] columns.

Instructions

The photos show the brown board freely pivoting on the thin rod, which has one end clamped between holding fixtures on the tall tripod stand.

The plumb bob hung from the thin rod shows the vertical line through the pivot point, which obviously intersects the two painted lines from previous trial runs on the board. One may also just hand-hold the thin rod for simplicity of setup.

One notes that the intersection of the two white lines is at the center of mass.

Now one demonstrates that when the entire board is held horizontally in one hand, that hand may be replaced by a single upturned finger tip (of the other hand ) applied on the bottom of the board at that point.

Writeup created by David A. Burba

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