If you missed today 2012-2013: We did day 1 of 2 on center of mass and stability.
The center of mass of an object is a point (no width, no length, no thickness; only position) where all the mass of the object can be considered to be.
Our virtue was trustworthiness.
We did a stability demonstration where we had a nail pounded in the center of a uniform density treated short piece of 2X4 with a string plumb bob hanging from the nail. I also placed sand paper at the base of the 2X4 piece so it would not slide as I tilted a second board it was resting on. As I lifted one end of the board the 2X4 was resting on the string indicated where the center of mass was in relation to the base and as soon as the string was not over the base the 2X4 fell over.
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So an object is stable as long as its center of mass is over its base.
The two ways of making an object more stable is:
1. Make the base larger.
2. Lower the center of mass so it is closer to the base.
2. Lower the center of mass so it is closer to the base.
Examples:
1. Wrestlers will widen their stance to make a bigger base and bend their knees to lower their center of mass in order to be more stable.
2. I had a small garbage can between the front seats in our van that kept falling over. I could either lower the center of mass of the garbage can by putting a small plastic bag of sand or rocks at the base OR attach the base to a board that was longer and wider than the base. I attached the thin board so I wouldn't add more weight to the vehicle so my gas mileage would not get worse.
2. I had a small garbage can between the front seats in our van that kept falling over. I could either lower the center of mass of the garbage can by putting a small plastic bag of sand or rocks at the base OR attach the base to a board that was longer and wider than the base. I attached the thin board so I wouldn't add more weight to the vehicle so my gas mileage would not get worse.
Hallway Physics
Then we all went out in the hallway and tried these center of mass/stability demonstrations:
1). with our back against the wall we tried to lean over and touch our toes. Since our center of mass is just below our naval, as soon as our center of mass was over our toes we fell over....try it.
2). with either shoulder against the wall and the inside side of our foot against the wall we attempted to raise our outside foot.
3). facing the wall with our toes against the wall and our nose against the wall, we tried to stand on our toes.
1). with our back against the wall we tried to lean over and touch our toes. Since our center of mass is just below our naval, as soon as our center of mass was over our toes we fell over....try it.
2). with either shoulder against the wall and the inside side of our foot against the wall we attempted to raise our outside foot.
3). facing the wall with our toes against the wall and our nose against the wall, we tried to stand on our toes.
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Now since most females carry their more mass in their hips and males carry more mass in the upper body the females center of mass is a little lower than a males and thus females are more stable. The following two demos the females could do where the males could not do them.
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4. kneel down and place your elbow against your knee with your fingers extended and place an object (we used a white board marker, a pack of gum, a small box of chalk, etc.) upright that distance from your knee. Then cross both hands behind your back and lean forward to knock over the object with your nose.
5. place your feet two of your feet away from a wall then lean over a chair or stool with your head resting against the wall and pick up the chair or stool, then stand up.
5. place your feet two of your feet away from a wall then lean over a chair or stool with your head resting against the wall and pick up the chair or stool, then stand up.
Other stability tricks that just some students attempted
6. Stand on your head with your hands down also by squatting down so your knees rest on your elbows and tipping your head to the floor as your knees stay on your elbows and then throw your feet into the air and stand on your head as long as you can.
7. Face your opponent and step your right feet forward next to each other. Shake hands and place the other foot where ever you feel most stable. Now if you get your opponent to move either foot before they get you to move yours you win. It is a stability game....notice the line your feet our in....just pull your opponent forward perpendicular to the line your feet our in and quickly, when (and if) they push or push your opponent back perpendicular to the line of your feet.
7. Face your opponent and step your right feet forward next to each other. Shake hands and place the other foot where ever you feel most stable. Now if you get your opponent to move either foot before they get you to move yours you win. It is a stability game....notice the line your feet our in....just pull your opponent forward perpendicular to the line your feet our in and quickly, when (and if) they push or push your opponent back perpendicular to the line of your feet.
Then we watch a video clip in True Lies where the pelican lands on the hood of a teetering vehicle causing it to fall and then the Myth-Busters version of the same stunt.
Finally we watched as two funnels taped together at their larger opening rolled uphill on two meter sticks. Actually the center of mass of the two funnels rolled downhill is why it works.
Then we each obtained a meter stick and balanced it at the 50 cm mark on our finger. Then we set the meter stick on a finger of each hand and moved our fingers together to locate the center of mass. Then we did the same thing only we just moved our left finger toward our right finger without moving our right finger and we were still able to locate the center of mass of the object. Then a student took a long metal bar that was much heavier at one end and tried the above showing the center of mass was closer to the heavier end.
Then we each obtained a meter stick and balanced it at the 50 cm mark on our finger. Then we set the meter stick on a finger of each hand and moved our fingers together to locate the center of mass. Then we did the same thing only we just moved our left finger toward our right finger without moving our right finger and we were still able to locate the center of mass of the object. Then a student took a long metal bar that was much heavier at one end and tried the above showing the center of mass was closer to the heavier end.