Motion/C.M of compsite objects
Concepts to investigate:
Composite bodies,center of mass,center of mass below point of support,stability.
Belt,piece of wood,saw,hammer,hinge,boards,clamp,potato,tooth-pick,two heavy forks.
Principles and procedures:
Part 1: Center of mass above the base of support.
If two or more objects are fastened to each other,a composite body is formed with a center of mass of its own. Cut a notch into the block of wood. Place the block of wood onto the table and locate the position of the center of mass by moving the block to the last point at which it will remain balanced. Mark this point on the block. By adding another object on the block,it is possible to shift the center of mass. Place a belt in the slot and after positioning it so it once again balances,place another mark on the wood to see where the new center of mass is.
How does the belt affect the center of mass of the composite object?
Part 2: Center of mass below the base of support.
Cut a small piece from the end of a potato and stick a toothpick in its centre. Try to balance the potato on the top of a post or inverted glass. It is difficult because the center of mass is above the center of support and the base of support (diameter of tooth-pick)is very small . Now stick two forks in the potato such that the majority of the fork is below the bottom of the tth picks and again try to balance the potato,adjusting the forks if necessary. The potato balances because the center of mass has been shifted and now below the point of support. When the center of mass is below the point of support,the object is stable. After any disturbances,the object rotates so that the center of mass returns to its position under the point of support. Grasp one of the forks and give it a slight push. The center of mas will move but gravity will then pull it down to its original position under the point of support.
A board attached to another board by a hinge will rotate downwards when there is no weight added,if however an object is added to the freely moving board such that the center of mass for the new composite body is inside and below the pivot point,the reverse will happen. Using a rope or strap,position a hammer so that it is supported by the by the board. The board remains stationary because the center of mass is directly below the point of support. Consequently there is no net torque on the composite body. Now move the hammer so that the center of mass is inside and below the point of support. The board now rotates upwards because there is a net upward torque on the composite body. Reposition the hammer so that the center of mass is outside and below the hinge,the board rotates downwards because there is a net down ward torque on the composite body.