Mathematical and Physical Journal
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# Problem P. 4207. (December 2009)

P. 4207. Two tennis balls of mass 60 g are attached with a massless rubber thread, and held in the vertical position as shown in the figure. In this position the unstretched length of the rubber thread is 40 cm. The upper ball is slowly raised vertically upward, until the lower ball just becomes unsupported by the ground. At this time the length of the thread is 1 m. The rubber thread exerts a force which is proportional to its extension.

a) How much work is done while the upper ball was raised?

b) Releasing the upper ball at what speed will it hit the lower one?

c) How much time elapses between the release of the upper ball and the collision?

(5 pont)

Deadline expired on January 11, 2010.

Sorry, the solution is available only in Hungarian. Google translation

Megoldás. $\displaystyle a)$ 0,53 J.

$\displaystyle b)$ 5,1 m/s.

$\displaystyle c)$ 0,34 s.

### Statistics:

 138 students sent a solution. 5 points: Batki Bálint, Béres Bertold, Bodosi Eszter, Bolgár Dániel, Börcsök Bence, Budai Ádám, Farkas Martin, Fülep Andrea , Galzó Ákos Ferenc, Hartstein Máté, Havlik Tamás, Hoksza Zsolt, Illés Dorottya, Jéhn Zoltán, Kaposvári István, Kovács 255 Márton, Kungl Ákos Ferenc, Kunsági-Máté Sándor, Laczkó Zoltán Balázs, Mayer Martin János, Nagy 668 Balázs, Pálovics Péter, Para Attila, Pataki Bálint Ármin, Patartics Bálint, Réti Dávid, Sisák Mária Anna, Szabó 928 Attila, Tamás Zsolt, Tamási Mátyás, Várnai Péter, Vuchetich Bálint, Wang Daqian, Wirnhardt Bálint, Zsolczai Viktor. 4 points: 34 students. 3 points: 37 students. 2 points: 11 students. 1 point: 10 students. 0 point: 9 students. Unfair, not evaluated: 2 solutions.

Problems in Physics of KöMaL, December 2009