Correct Use of the Parallel Axis Theorem for Moment of Inertia

Click For Summary
The discussion focuses on the correct application of the Parallel Axis Theorem for calculating the moment of inertia. The user initially calculates the moment of inertia for a rod in two cases but arrives at incorrect values, particularly for the second case with arms stretched. The error stems from not properly applying the theorem, specifically neglecting to use the moment of inertia about the center of mass. After clarification, the user acknowledges the mistake and realizes the need to incorporate the correct formula. Accurate application of the theorem is essential for obtaining the correct moment of inertia values.
simphys
Messages
327
Reaction score
46
Homework Statement
In order to increase her moment of inertia about a vertical axis, a spinning figure skater stretches out her arms horizontally; in order to reduce her
moment of inertia, she brings her arms down vertically along her sides. Calculate the change of moment of inertia between these two configurations of the arms. Assume that each arm is a thin, uniform rod of length 0.60 m and mass
2.8 kg hinged at the shoulder at a distance of 0.20 m from the axis of rotation.
Relevant Equations
xD
so calculated, the moment of inertia for a rod about an axis at the end of the rod is I = 1/3 * M * L^2
here for case 1: arms to the side
I is calculated to be ##I = 0.224##

for case 2: arms stretched
## I = 1 / 3 * M * L^2 + M * d^2 ## with L = 0.6 m (length of rod) and d = 0.2 (dinstance from 'hinge' to axis of rotation)
## I = 0.896## is my answer.
Yet the actual answer is ##I = 1.568##. I don't really understand what I am doing wrong here so can someone help me please?

Reasoning:
In case 2 I basically have the moment of inertia at the end (the 'hinge'/shoulder so to say) and need to add a dinstance d to get the moment of inertia about the rotation axis via the parallel axis theorem yet this isn't correct.

Thanks in advance
 
Physics news on Phys.org
simphys said:
for case 2: arms stretched
## I = 1 / 3 * M * L^2 + M * d^2 ## with L = 0.6 m (length of rod) and d = 0.2 (dinstance from 'hinge' to axis of rotation)
## I = 0.896## is my answer.
You have not used the parallel axis theorem correctly. $$I = I_{\rm cm} + md^2$$ What does ##I_{\rm cm}## mean?
What does ##d## mean?
 
  • Like
Likes simphys and hutchphd
TSny said:
You have not used the parallel axis theorem correctly. $$I = I_{\rm cm} + md^2$$ What does ##I_{\rm cm}## mean?
What does ##d## mean?
yeah apologies it should have been 2 times the symbolic equation, just forgot to type it out.
Omg... totally forgot that... thanks a lot... I needed to use the moment of inertia about the center of mass basically..,.
 
If have close pipe system with water inside pressurized at P1= 200 000Pa absolute, density 1000kg/m3, wider pipe diameter=2cm, contraction pipe diameter=1.49cm, that is contraction area ratio A1/A2=1.8 a) If water is stationary(pump OFF) and if I drill a hole anywhere at pipe, water will leak out, because pressure(200kPa) inside is higher than atmospheric pressure (101 325Pa). b)If I turn on pump and water start flowing with with v1=10m/s in A1 wider section, from Bernoulli equation I...

Similar threads

  • · Replies 28 ·
Replies
28
Views
1K
Replies
11
Views
3K
  • · Replies 12 ·
Replies
12
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
Replies
25
Views
2K
  • · Replies 21 ·
Replies
21
Views
2K
  • · Replies 8 ·
Replies
8
Views
956
Replies
8
Views
2K
  • · Replies 3 ·
Replies
3
Views
1K
  • · Replies 3 ·
Replies
3
Views
1K