Go Back   Physics Help Forum > College/University Physics Help > Advanced Mechanics

Advanced Mechanics Advanced Mechanics Physics Help Forum

Reply
 
LinkBack Thread Tools Display Modes
Old Sep 4th 2008, 08:00 AM   #1
Junior Member
 
Join Date: Sep 2008
Posts: 7
Exclamation Another rotational motion problem.

An overhead view of a ring that can rotate about its center like a merry go round. Its outer radius R is 0.8 m, its inner radius r is 0.4 m, its mass is 8 kg, and the mass of the crossbars in the ring is negligible. It initially rotates at an angular speed of 8rad/s with a cat of mass 2 kg on its outer edge, at radius R. By how much does the cat increase kinetic energy of the cat-ring system if the cat crawls to the inner edge, at radius r? (The answer is 39.1J --- From textbook)

I don't know the solution.

I tried to find initial and final moment of inertia (I)

I-initial = (0.5)(8)((0.8^2)+(0.4^2)) + 2(0.8^2) = 4.48 kgm^2

I-final = (0.5)(8)((0.8^2)+(0.4^2)) + 2(0.4^2) = 3.52 kgm^2

Then I don't know how to do it.

Please help.
Thank you.
noppawit is offline   Reply With Quote
Old Sep 4th 2008, 08:28 AM   #2
Forum Admin
 
topsquark's Avatar
 
Join Date: Apr 2008
Location: On the dance floor, baby!
Posts: 2,468
Originally Posted by noppawit View Post
An overhead view of a ring that can rotate about its center like a merry go round. Its outer radius R is 0.8 m, its inner radius r is 0.4 m, its mass is 8 kg, and the mass of the crossbars in the ring is negligible. It initially rotates at an angular speed of 8rad/s with a cat of mass 2 kg on its outer edge, at radius R. By how much does the cat increase kinetic energy of the cat-ring system if the cat crawls to the inner edge, at radius r? (The answer is 39.1J --- From textbook)

I don't know the solution.

I tried to find initial and final moment of inertia (I)

I-initial = (0.5)(8)((0.8^2)+(0.4^2)) + 2(0.8^2) = 4.48 kgm^2

I-final = (0.5)(8)((0.8^2)+(0.4^2)) + 2(0.4^2) = 3.52 kgm^2

Then I don't know how to do it.

Please help.
Thank you.
Rotational kinetic energy is
$\displaystyle K = \frac{1}{2}I \omega ^2$

-Dan
__________________
Do not meddle in the affairs of dragons for you are crunchy and taste good with ketchup.

See the forum rules here.
topsquark is offline   Reply With Quote
Reply

  Physics Help Forum > College/University Physics Help > Advanced Mechanics

Tags
motion, problem, rotational



Thread Tools
Display Modes


Similar Physics Forum Discussions
Thread Thread Starter Forum Replies Last Post
rotational motion Ellekaycee Advanced Mechanics 1 Mar 19th 2013 02:54 PM
Rotational Motion Sheldon Kinematics and Dynamics 2 Nov 5th 2012 09:29 AM
Rotational Motion truho Kinematics and Dynamics 1 Apr 7th 2012 10:54 PM
Rotational Motion Problem .daniel Kinematics and Dynamics 8 Nov 29th 2011 01:49 PM
rotational motion hikage Advanced Mechanics 2 Jun 5th 2009 04:00 PM


Facebook Twitter Google+ RSS Feed