Go Back   Physics Help Forum > High School and Pre-University Physics Help > Periodic and Circular Motion

Periodic and Circular Motion Periodic and Circular Motion Physics Help Forum

Reply
 
LinkBack Thread Tools Display Modes
Old Jan 23rd 2010, 11:16 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 11:26 PM.
mephisto50 is offline   Reply With Quote
Old Jan 23rd 2010, 11:29 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
physicsquest is offline   Reply With Quote
Old Jan 23rd 2010, 11:35 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 04:20 AM.
physicsquest is offline   Reply With Quote
Old Jan 23rd 2010, 11:44 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
mephisto50 is offline   Reply With Quote
Reply

  Physics Help Forum > High School and Pre-University Physics Help > Periodic and Circular Motion

Tags
explanation



Thread Tools
Display Modes


Similar Physics Forum Discussions
Thread Thread Starter Forum Replies Last Post
Simple explanation needed jackthehat Kinematics and Dynamics 3 Apr 22nd 2015 09:01 AM
Energy Conversion Explanation rickymcferrin Energy and Work 1 Jul 25th 2012 09:32 AM
What is the true explanation for this? Misr Kinematics and Dynamics 0 Feb 18th 2010 06:45 AM
Explanation needed. tsmith Kinematics and Dynamics 1 Sep 12th 2009 10:58 AM
speed explanation chaoticphysics Kinematics and Dynamics 1 Mar 31st 2009 04:35 PM


Facebook Twitter Google+ RSS Feed