Reports of the Department of Mathematical Information Technology Series B. Scientific Computing No. B. 2/2011 On the Instability of Elastic Web Axially Moving in the Gravitational Field Nikolay Banichuk Juha Jeronen Tytti Saksa Tero Tuovinen University of Jyv ¨ askyl¨ a Department of Mathematical Information Technology P.O. Box 35 (Agora) FI–40014 University of Jyv ¨ askyl¨ a FINLAND fax +358 14 260 2731 http://www.mit.jyu.fi/