Mathematical and Physical Journal
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# Problem P. 4158. (April 2009)

P. 4158. A uniform solid disc of radius 10 cm and of mass 5 kg is rotated at an angular speed of 120 1/s along its horizontal symmetry axis, and carefully, without giving any initial speed to it, it is placed to a horizontal surface. The coefficient of friction between the disc and the surface is 0.4.

Determine the work done by the frictional force on the disc, while it covers a distance of 5 m:

a) with using the definition of work done;

b) with using the work-energy theorem.

(Neglect air resistance and rolling friction, and calculate with g=10 m/s2.)

(5 pont)

Deadline expired on May 11, 2009.

### Statistics:

 63 students sent a solution. 5 points: Bartha Zsolt, Boros Csanád Örs, Filep Tibor, Galzó Ákos Ferenc, Garaguly Gergő, Iván Dávid, Kaposvári István, Kungl 008 Ákos, Lovas Lia Izabella, Maknics András, Pálovics Péter, Patartics Bálint, Pázmán Koppány, Sós Attila Róbert, Szabó 895 Dávid, Trényi Róbert, Vuchetich Bálint, Zelei Kristóf. 4 points: Balogh Gábor, Barancsuk Lilla, Béres Bertold, Börcsök Bence, Budafoki Richárd Gergely, Budai Ádám, Egyed Zsombor, Farkas Martin, Farkas Márton, Földes Imre, Galgóczi Gábor, Jéhn Zoltán, Kovács Márk, Lőrincz Dóra, Márkus Bence Gábor, Nagy 111 Miklós, Palotás Julianna, Szük Dániel, Varju 105 Tamás, Wang Daqian, Zsámboki Richárd. 3 points: 12 students. 2 points: 8 students. 1 point: 1 student. 0 point: 1 student. Unfair, not evaluated: 2 solutions.

Problems in Physics of KöMaL, April 2009