Center of Mass Lesson

·        COLLECT HOMEWORK

·        CONCEPTS TO INVESTIGATE: 

o      Geometric Center, Internal Center of Mass, External Center of Mass.

o      Appropriate for any level science class.

·        PURPOSE:

o      It is intended that students gain an intuitive understanding of center of mass, center of gravity, and how they relate to daily life. 

·        OBJECTIVES (The student will be able to):

o      Estimate the internal or external center of mass for a given body.

o       Understand the difference between center of mass and center of gravity, as well as weight and mass.

·        ACTIVITIES: 

o      PRE-LAB:

§       The center of mass of an object is that point at which the object’s entire mass may be considered to be concentrated.  Students will be given various real life pictures and then asked to mark an “X” at the approximate location where they believe the object’s center of mass would be found.        

§       Center of mass of a meter stick.

§       Center of mass vs. Center of Gravity

§        View Sumo Wresting Picture

§        Sumo wrestling:

·        1500 years old

·        Samarai

·        jujitsu

·        Time of prosperity

o       Sumo becomes entertainment

·        Rule:  A bout is won by forcing the opponent out of the inner circle or throwing him in the doyho.

·        Where is the sumo wrestler’s center of mass?

·        Where is the sumo wrestler’s center of gravity?

·       MARK WITH AN “X” THE LOCATION OF THE WRESTLER’S CENTER OF MASS.

·       View Picture of a Pitt Bull Terrier    

o       Background Information:  It was the cross between the Bulldog and the terrier that resulted in the Staffordshire Terrier, which was originally called the Bull-and-Terrier Dog, Half and Half, and also the Pit Dog or Pit Bullterrier.  Until the early part of the 19th century, the Bulldog was bred in England for the purpose of bull baiting. Bull baiting was a cruel sport in which a tethered bull, brought to market for slaughter would be attacked by bulldogs with the notion that the meat would be tenderized, prior to slaughter, by the bulldog’s attacks. The contention that the meat was tenderized during these brutal bouts was not based on any fact and indeed served no real purpose other than to line the pockets of the men who trained their dogs to participate in these sadistic blood-matches. During these matches the dog would assault the bull, avoid the stomping hooves and slashing horns, grab on to a nose or ear, and hang on until the bull collapsed from exhaustion or lack of oxygen. Many of the dogs were crushed by stomping hooves, disemboweled by slashing horns, and tossed through the air causing broken legs, backs, and skulls when they hit the ground. On the whole, both the dogs and the bulls suffered greatly. Every class of person from commoners to royalty enjoyed these spectacles as a means to divert themselves from the violence and diseases of their day to day lives.  Mass public outcry finally forced Parliament to take a stand and ban the practice of bull baiting in 1835.  Once bull baiting was banned, dog breeders who appreciated the fierceness, courage, and tenacity of the bull dogs turned their attentions to breeding dogs for the purpose of dog fighting. They began with the bull dog, mixed in some terrier blood for gameness, and produced the Bull and Terrier, a dog that met all of their expectations. The Bull and Terrier was bred for aggression to other dogs, unrelenting bravery, a high pain threshold, a superior blood clotting ability to aid him when wounded, a willingness to fight to the end, and an unmatched affection for people. These attributes were exploited by man, and these dogs were encouraged to fight each other in pits. The outcome of these matches depended upon the challenge issued. Sometimes the fight lasted until one dog was either pinned or chased from the pit, and other times the fight was not to be stopped until one of the dogs was dead. In any case, this is an example of man abusing the gift of the dog as a loyal companion by allowing these animals to suffer injury in order to gain the praise of their masters. An interesting fact to mention here is that the original breeders of these fighting dogs held one attribute to be absolutely necessary, devotion to people. If a pit dog ever turned on, bit, or showed aggression to a human it was put down immediately. This led to the breeds overwhelming people friendly personality and truly kept the breed from being an outright danger to man. (Source=http://www.geocities.com/Heartland/Pointe/8959/History_of_the_Breed.html)

·       IN TERMS OF CENTER OF MASS AND CENTER OF GRAVITY, WHAT MAKES THE PIT BULL TERRIER SUCH A GOOD FIGHTING DOG?

·        MARK WITH AN “X” THE LOCATION OF THE DOG’S CENTER OF MASS

·       View a Tall redwood Tree

o       On December 10th, 1997, 23 year old Julia Butterfly Hill climbed 180 feet up an ancient redwood she calls Luna and has not come down. In the American tradition of civil disobedience, she is conducting her courageous vigil on behalf of one of the last wild places left in our country, the remaining 3% of our magnificent old growth forests.

o       The cutting of the world’s tallest and biggest trees has divided California since 1900. Logging corporations have cut about 97 percent of the ancient coastal redwood forest that once covered two million acres. Hillsides once cloaked in cathedral forests are littered with stumps the size of small cars--testament to the cutting of wood for uses as diverse as Victorian-era houses in San Francisco and modern-day decks and hot tubs.

o       Julia Butterfly Hill is the latest incarnation of a California protest tradition dating to 1925, when "lady conservationists" first thwarted loggers by blocking access to redwood groves. Nowadays, treesitting is a standard protest technique and Julia’s tree sit essentially began in October when, under a full moon, a team of activists hiked up a ridge near the town of Stafford to the largest remaining redwood near a clearcut-caused landslide that wiped out 7 homes. Assembling the platform in the moonlight, they named the tree "Luna."

·       MARK WITH AN “X” THE APPROXIMATE LOCATION OF THE CENTER OF MASS OF THIS TREE.

·        GIVE A REASON IN TERMS OF CENTER OF MASS WHY REDWOOD TREES ALWAYS SEEM TO GROW ALMOST STRAIGHT UP TOWARDS THE SKY WHILE SHORTER TREES TEND TO GROW AT AN ANGLE MORE OFTEN.

·       ( Source =  http://www.lunatree.org/)

·        Laboratory Exercise

o      From a piece of cardboard, cut out an irregular shape.  Use a nail or pencil to punch three holes in the perimeter of the cardboard.  Slightly enlarge these holes so the cardboard can rotate freely.  Place a nail or pencil in one hole and hang a weighted piece of string.  Swing the cardboard and when it comes to rest, draw a line on the cardboard showing the position of the string.  (The cardboard swings freely until the center of mass comes to rest directly below the point of support).  This position represents the state of least energy for the body.  (All bodies try to reach a state of least energy).  Repeat procedure for the two other holes.  Attempt to balance the cardboard at the intersection of the three lines.  (This should be possible if you have found the object’s center of mass).     

·       What is the fewest number of lines needed to locate an object’s center of mass?

·       Attempt to find the center of mass for a coat hanger.  (hint:  you may need to tape a piece of paper in the center of the hanger)

·       Geographic center of California.

·       Homework:  Find Examples of five things in real life where the center of mass lies outside the body itself.

·       MATERIALS:  Cardboard, nails, tape, coat hangers, map of California, scissors.