Hibbler dynamics problem 13-97

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Discussion Overview

The discussion revolves around a dynamics problem involving a smooth particle attached to an elastic cord, focusing on calculating the force exerted by the guide on the particle. The problem includes elements of angular motion and forces in a horizontal plane, with specific parameters provided for mass, cord characteristics, and motion path.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Technical explanation

Main Points Raised

  • The initial attempt at a solution calculates the force produced by the cord as 6.64 Newtons, based on the difference in lengths and the spring constant.
  • Participants suggest that the correct answer is 7.67 Newtons, indicating a need to consider tangential and normal forces.
  • A participant advises checking the calculations and recommends drawing a free body diagram (FBD) to clarify the forces acting on the particle.
  • Equations of motion are proposed, incorporating radial and transverse components to solve for the unknown forces.
  • A later post claims to derive the correct force of 7.67 Newtons through a detailed solution process involving equations of motion.

Areas of Agreement / Disagreement

There is no consensus on the initial calculation of the force produced by the cord, with participants presenting different values and approaches to arrive at the solution. The discussion remains unresolved regarding the correctness of the initial attempt versus the later calculations.

Contextual Notes

The discussion highlights potential limitations in the initial approach, such as the need for a clear coordinate system and the correct application of forces in the FBD. There may be unresolved assumptions regarding the dynamics of the system.

Who May Find This Useful

This discussion may be useful for students or individuals studying dynamics, particularly those interested in problems involving forces, motion in a circular path, and the application of elastic properties in mechanics.

nko315
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Homework Statement


Smooth particle with mass of 80 grams is attached by an elastic cord from O to P. O is at the base of a circle with radius .4m. Angle theta is 60deg from horizontal. The cords unstretched length is .25m. K of the cord is 30N/m
The path the particle makes is r=0.8sin(theta)m. The constant angular velocity is 5 rad/s. An arm moves this particle.


Homework Equations


What I am looking for is the force that the guide has on the particle. This particular one is in the horizontal plane so gravity is out. How do you do this?


The Attempt at a Solution


Force that the cord produces is (0.69m-.25m)30N/m or 6.64 Newtons
The listed answer is 7.67N. I know this has something to do with the tangential and normal forces due to the particle itself.
 
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Can you post a diagram of the problem>?
 
Last edited:
Drawing for Hibbler 13-97

I attached a word drawing of the dynamics problem. Hopefully it will make the problem clearer.
Thanks
 

Attachments

you should check your math on the force that the cord producesGo back to the basics, and draw a FBD of the particle with all the forces acting on it,

establish a coordinate system for the radial and transverse components

(hint: the elastic cord's force is collinear with the radial component, and the guide's force is collinear with the transverse component)

once you have this FBD, it should be easy to write down your equations of motion to solve for the unknowns
 
Last edited:
Solution. Woo hoo

Npsin(θ ) − 30N/m(r − .25m) = .08kg(r'' − rθ'2)
F−Npcos(θ ) = .08kg(rθ'' + 2r'θ')

If you work this all the way out it finally does work out to 7.67N
 

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