Moment of Inertia Problems and Conservation of Energy

Click For Summary

Homework Help Overview

The discussion revolves around a problem involving the moment of inertia of a pulley and the application of conservation of energy to determine the speed of a mass as it falls. Participants are exploring how to relate the mechanical energy of the system to the motion of the masses and the pulley.

Discussion Character

  • Exploratory, Conceptual clarification, Mathematical reasoning

Approaches and Questions Raised

  • Participants discuss the application of conservation of energy principles, questioning how to set up the energy equations for the system. There is a focus on comparing the total mechanical energy before and after the mass falls, and how to account for the energies of both masses and the pulley.

Discussion Status

Some guidance has been offered regarding setting up a conservation equation that includes kinetic and potential energy terms for all components of the system. However, there is still uncertainty among participants about how to express these terms and the implications of using conservation of energy versus kinematics.

Contextual Notes

Participants are working under the assumption that they have the necessary mass and radius values for the objects involved, but there is an indication of confusion regarding the initial and final energy states of the system.

Lancelot59
Messages
640
Reaction score
1
I have this situation here:

GIANCOLI.ch10.p067.jpg


I'm given the masses for the objects, as well as the radius and mass of the pulley. I have to use conservation of energy laws to find out how fast the mass on the right is going the instant it hits the ground.

I can find out what the alpha of the pulley is, and therefore the accelration of the system, but that's just kinematics. How can I use conservation of energy to do it?

How could I do it if the pulley was ideal?
 
Physics news on Phys.org
Lancelot59 said:
How can I use conservation of energy to do it?
Compare the total mechanical energy of the system before and after the mass falls.
 
I don't follow...

I see that there is none before, and some after, when the block b has zero potential energy, block A has potential and kinetic energy, and the pulley has rotational kinetic energy...but if I knew those values then I could just save the trouble and use kinematics.
 
Set up a conservation equation:
KE1 + PE1 = KE2 + PE2

Make sure you include the PE and KE of both masses and the pulley. You'll be able to solve for the speed of block B without using force equations or kinematics. Hint: Express all the KE terms in terms of the speed of block B.
 

Similar threads

Moment of Inertia of something that rotates
  • · Replies 15 ·
Replies
15
Views
2K
Understanding Kinetic Energy: Moment of Inertia and Rotational Motion
  • · Replies 5 ·
Replies
5
Views
2K
Moment of Inertia of an Ammonium Molecule
  • · Replies 3 ·
Replies
3
Views
4K
Calculating Moment of Inertia for Hollow Cylinder w/ Water
  • · Replies 40 ·
2
Replies
40
Views
6K
What is the "r" in the moment of inertia?
  • · Replies 13 ·
Replies
13
Views
3K
Calculating Flywheel Inertia Using Conservation of Energy Equations?
  • · Replies 14 ·
Replies
14
Views
2K
Potential difference defined in non-conservative electric field
  • · Replies 12 ·
Replies
12
Views
2K
Plane pendulum: Lagrangian, Hamiltonian and energy conservation
  • · Replies 6 ·
Replies
6
Views
4K
Moment of inertia of a composite object in Solidworks
  • · Replies 10 ·
Replies
10
Views
2K
Undergrad Moment of inertia of human body with limbs at an angle
  • · Replies 12 ·
Replies
12
Views
2K