Mathematical and Physical Journal
for High Schools
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# Problem P. 4149. (March 2009)

P. 4149. A 25 cm-long simple pendulum is hanging vertically inside a small car which is at rest. The mass of the bob is 2 grams, its charge is 5.10-8 coulomb. The vehicle is in uniform electric field, and then it will travel in this field. The electric field points vertically upward and its magnitude is 400 kN/C. The vehicle is started at an acceleration of 4.5 m/s2. Air resistance is negligible.

a) Find the maximum speed of the bob with respect to the vehicle.

b) What must the strength of the cord be in order not to be broken during the motion.

(5 pont)

Deadline expired on April 14, 2009.

### Statistics:

 60 students sent a solution. 5 points: Béres Bertold, Bolgár Dániel, Boros Csanád Örs, Börcsök Bence, Budafoki Richárd Gergely, Csányi Gergely, Deák Zsolt, Farkas Martin, Farkas Márton, Filep Gábor, Filep Tibor, Fonai Dániel, Földes Imre, Hartstein Máté, Hegedűs Csaba, Iván Dávid, Jakab Dávid, Jéhn Zoltán, Ji Hai Ou, Kovács 235 Gábor, Kovács Benjámin, Kovács Márk, Kungl 008 Ákos, Lászlóffy András, Lovas Lia Izabella, Lőrincz Dóra, Mayer Martin János, Nagy 111 Miklós, Pálovics Péter, Patartics Bálint, Pázmán Koppány, Rácz Viktor, Szabó 928 Attila, Tóth 222 Barnabás, Trényi Róbert, Varju 105 Tamás, Vuchetich Bálint, Wang Daqian. 4 points: Barancsuk Lilla, Bartha Zsolt, Csengeri Béla, Krämer Zsolt, Laczkó Zoltán Balázs, Maknics András. 3 points: 5 students. 2 points: 1 student. 1 point: 5 students. 0 point: 2 students. Unfair, not evaluated: 3 solutions.

Problems in Physics of KöMaL, March 2009