Center of Mass

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2.1) The Windmill

Chelsea is practicing her break dancing moves for a "battle" that she will have with her next-door neighbor Arvi later in the afternoon. She has been working on the "windmill" move, in which she spins on her back and shoulders with her legs in the air, but she is having trouble keeping her turns tight about the axis of her shoulders and neck. In the language of physics, why might this be? *

2.2) Native Pottery

A Sioux Indian artist is hand crafting a traditional clay pot alongside other Lakota artists at their plant in Rapid City, South Dakota.  He molds the clay into a ball and drops it onto the spinning potter's wheel. The resulting shape of the clay is oblong, not round. What can we say about the distribution of mass when he dropped the clay onto the wheel? *

2.3) Off to Work

A mother who weighs 70 kg hugs her 10 kg child goodbye before she leaves for work in the morning and then hands the toddler to her husband who stays at home to watch the children and manage the house. If she is standing straight while holding the child out for her husband at what distance from her torso will their center of mass be? *

2.4) Keep it in the Family

Qinaluqana adjusts the belt around her waist that supports her 15-month-old niece who rests against her back. She wraps her light spring parka about them both and walks outside to join the rest of her Inupiat community as it helps to divide the rewards of a recent hunting expedition. Estimating heights and weights, where do you think the center of mass is located on this aunt-niece system?**

2.5) Baby Shower

A woman bends her knees and reaches across for a 15 lb sack of charcoal briquettes she is purchasing for her friend's BarBQ baby shower. When she is stooped and holding the bag away from her body, where is the center of mass of the 'system'? Will she be able to balance in this position? **

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2.6) Baby Mobile

Rheanna is using some of her old stuffed animals to make a mobile to hang over her baby sister’s crib. She has a teddy bear (15 g), a lamb (17 g), a puppy (21 g) and a seal (14 g). She also has 3 small dowels, each 15cm long with a mass of 4 g, and some thread of negligible mass. She wants to hang the bear and the puppy from the ends of one dowel, the seal and the lamb from the ends of the other, suspend each from the ends of the third dowel, and hang the whole thing from the ceiling. Explain where on each dowel the threads should be attached so the whole thing hangs level and looks nice. **

# Center of Mass Lab

2.7) Long-boarding Lab

Shannon Dorato

Imagine you and a friend are riding on a long-board with a mass of 9 kg and length of 1.3m. One of you will be standing on the back of the board pumping, while the other is sitting on the front of the board steering. Now back in the lab, measure your weights and with a board and a scale determine your centers of mass. Measure the board and label the middle with a mark. Place one end on the scale and the other on a block so it will be level. Lay flat along the board, adjusting your position until the scale reads half of your normal weight. The halfway mark on the board will be directly in line with your center of mass. Measure the displacement of the center of mass from the ground for both a standing and squatting person. Compare these displacements when added to the displacement of the longboard. Which one will make a tighter turn? Discuss.

Momentum Problem Sets

Posted on 8/1/05