Motion/Stability

Concepts to investigate

Stability,base of support,center of mass.

Materials

Large rectangular blocks of wood,nails,washer,paper,scissors,Erlenmeyer flask,salt,straw,ball.

Principles and and procedures

The ball(sphere) demonstrates neutral stability in that its center of mass always remains over the point of support when the abject is resting or moving on a level surface

When the cone is resting on its sides ,it also demonstrates neutral stability in that it will remain in any position placed. The cone is stable on its bottom base and unstable on its top. Try balancing the tip of the cone or the tip of a pencil on your finger. It is not easy because the slightest motion will cause the center of mass to fall outside the point of support and the object will topple.

You may have seen a person walking a tight rope and swinging his or her arms to maintain balance. Indeed the whole idea about balancing is to keep the center of mass above the point of support. The acrobat moves his or her arms to keep the center of mass over the point of support(rope)

Center of mass of humans:

Animal such as cows and horses learn to walk soon after birth while human require about a year. The center of mass of humans is in the abdomen and when one stands upright,the abdomen must be carefully positioned over the narrow support base,the two feet. By contrast the center of mass of four legged animals,is much lower and the base of support is much wider since there are four feet rather than of two. Given these facts,it is obvious that a four legged animal should have an easier time learning to walk and maintaining their stability.

Place pencil on the floor near your feet and attempt to pick it without bending your knees or moving your feet. Most people can do it. Now stand with the back of your shoes touching the wall,and try to pick up the pencil in the same way. You probably will not be able to pick it.When standing up ,your center of mass lies near your waist above your feet,which are the supporting base. When you bend over with your feet away from the wall, you are able to lean your legs and buttocks backwards,shifting the center of mass backwards and allowing you to compensate for any forward movement of the center of mass resulting from the forward movement of the top part of your body.

Balancing an object over a period of support:

An erlenmeyer flask can be balanced on an edge if the center of mass is directly above the point of support. Fill the flask approximately two-thirds full of water. Pour a pile of salt over the table top and then balance the flask,using the salt as support. Place a rag below the flask to soften the impact if it topples. The cubic crystals of salt brace the flask the same way that wooden blocks placed in front of car tires brace the tires and prevent the car from rolling.Once the flask is balanced ,use a straw to blow the salt away beneath the flask. It is fairly easy to blow away the salt except two crystals,on each side of the flask. Keep trying until it balances with just two crystals. If there are no vibrations,the flask can stand in this position for days.

Questions:

1. When you carry a large object at your side,you must lean in the opposite. Explain

2. In which sport is it particularly important to have a low center of mass?Why?

3. Why is a double deck bus more stable than a regular bus?

4. A flamingo has very long legs and a very long neck but is easily able to stand an d sleep on one leg. Explain.

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Answers:

1. You must must lean to the opposite side to ensure that the center of mass of the composite object(you and the object you are carrying) remains over the base of support(your feet).

2. In many combative sports(such as wrestling,boxing and football),it is important to keep a low center of mass so as to maintain stability when colliding with opponents.

3. A double-decker bus is less stable than a regular bus because,it has a high center of mass that is easily moved outside the base of support.

4. A flamingo moves its neck and feather the way an acrobat uses a balancing pole to position the center of mass over the point of support,its feet.


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