Physics Help Forum Need an explanation to this.

 Periodic and Circular Motion Periodic and Circular Motion Physics Help Forum

 Jan 23rd 2010, 10:16 PM #1 Junior Member   Join Date: Jan 2010 Posts: 3 Need an explanation to this. A particle is tied to an extensible string, which obeys Hooke's Law and performs circular motion with speed v and radius 2r on a horizontal smooth table. The natural length of the string is r. The speed of the particle is slowly increased such that the string's length becomes 3r. Find the new speed of the particle. edit: upon reading the sticky.. here's my attempt Applying F=-kx F=mv^2/r when speed=v, radius = 2r F=mv^2/2r F=-k(2r-r) when radius =3r, F=mv^2/3r F=-k(3-r) Here's where I'm stuck. Last edited by mephisto50; Jan 23rd 2010 at 10:26 PM.
 Jan 23rd 2010, 10:29 PM #2 Physics Team   Join Date: Feb 2009 Posts: 1,425 You have to post your attempt at solving the question before we can help. Heres a hint Try equating the centripetal force to the force from hookes law due to the extension
 Jan 23rd 2010, 10:35 PM #3 Physics Team   Join Date: Feb 2009 Posts: 1,425 Good. You have practically solved it. Find k in terms of v and then substitute in the 2 nd eqn (as k doesnt change). You should get the new vel in terms of the old v. Last edited by physicsquest; Jan 24th 2010 at 03:20 AM.
 Jan 23rd 2010, 10:44 PM #4 Junior Member   Join Date: Jan 2010 Posts: 3 Thanks! I realised using a capital V for the second equation helps to prevent confusion between the 2 different speeds. so here's my solution from eq 1: k=mv^2/2r^2 from eq 2: 2k= mV^2/3r^2 substituting k into 2nd equation, mv^2/r^2 = mV^2/3r^2 3v^2=V^2 V= root(3) v

 Tags explanation

 Thread Tools Display Modes Linear Mode

 Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post jackthehat Kinematics and Dynamics 3 Apr 22nd 2015 08:01 AM rickymcferrin Energy and Work 1 Jul 25th 2012 08:32 AM Misr Kinematics and Dynamics 0 Feb 18th 2010 05:45 AM tsmith Kinematics and Dynamics 1 Sep 12th 2009 09:58 AM chaoticphysics Kinematics and Dynamics 1 Mar 31st 2009 03:35 PM