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Momentum is conserved in the absence of external forces.  Its usefulness as a concept depends on our ability to designate a “system” which is relatively isolated from its surroundings.  In the following problems, explain what the system is, and describe its interactions with the external world.  Describe the internal interactions during which momentum is (approximately) conserved.  Sketch a diagram of the system before and after the interaction, labeling masses and velocities. 

1.1) Hackey-Sack

You and a bunch of friends circle around to play  Hackey-Sack. Each of you takes care to pass and get equal time handling the hackey-sack. *

1.2) Grounds Crew

Grounds Crew
Angie Cahill

A grounds crew has put forth a good effort over the last few days, to help restore a historical landmark. Melissa picks up the last bag of ivy she has cleared, swings her arm back, and releases the bag. As it hits the deck of the flatbed truck it knocks into another bag of leaves, which jumps forward as a result of the interaction. *  


1.3) Dreidel

Ray and Robin Parrott

It is the holiday season. Because the snow is falling outside so heavily, Rebecca and her family have decided to celebrate the first night of Chanukah with a game of Dreidel. The game is intense with competition between Rebecca and her little brother David, and when she wins his last chocolate coin he flicks it at her with a mock sigh of contempt. The coin slides straight across the table at a constant speed and collides with one of her gold coins of the same size. *

1.4) Seal Meal
© Australian Antarctic Division 2002, Kingston, Tasmania 7050

Akuvaaq steers his traditional skin covered umiak, jostling between the breaking ice of the ocean just off the coast of Point Barrow, Alaska. It is the end of June, a good time for hunting seal, and several of his Inupiat friends float alongside him in hopes of catching a meal. Suddenly, the glass surface of the water is shattered and they watch as their prey surfaces. Akuvaaq pulls back his arm and launches his spear at the seal. As he does, his umiak slides backwards. *

1.5) Soccer

While practicing soccer in an irrigation ditch, Mia kicks the ball at a 30-degree angle against the side of the ditch that is angled at 45 degrees. **  


1.6) Shichi-go-san

It is November 15th, the day on which the Japanese celebrate Shichi-go-san, a holiday for the health and longevity of their young children. Mitsuko Kuwahara has dressed her three-year-old daughter Yumi in her most vibrant kimono and brought her to the Shinto shrine to receive the blessings of the Kami. First the mother rings the gong producing a long low chime. Then she sets little Yumi-chan on the ground. The child bows and then swings the rope to hit the gong. The gong recoils slightly as it rings. **

1.7) Splashing Around

Water Drop
Adam Hart-Davis

Shaniqua and Kaesha are huddled over a small puddle. They are holding a palmful of gravel each and take turns dropping one stone at a time, marveling at the splash each stone creates. **  


1.8) Seed Spitting

Group Spitting Watermelon Seeds
Brian White

You and a group of friends are having a competition to see who can spit watermelon seeds the farthest. You simultaneously take a deep breath and with the seed pressed between your lips, send each out with a burst of lungpower. **

1.9) Rafting

Juan and Mildred are sending their pet grasshopper down the river to explore. For its travels they have constructed a simple boat from a dried Oak leaf. They give the boat a little shove and watch as their tiny friend drifts down the river and then swirls in an eddy only to be caught on a twig. **


Momentum Problem Set


Posted on 8/1/05

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