Calculating Polar Moment of Inertia for Automobile Wheel and Tyre

In summary, the conversation is discussing a problem involving an automobile wheel and tire suspended by a steel rod in the horizontal plane. The goal is to calculate the polar moment of inertia of the wheel and rod using various equations involving torque, torsional deformation, and shear modulus. The main point of confusion is how the number of oscillations and the period factor into the equations. Additional clarification is provided on the relevant equations and variables, and the importance of distinguishing between the mass and area polar moments of inertia is emphasized.
  • #1
a3sportback54
4
0

Homework Statement



I'm having a bit of difficulty with this question on SHM:

"An automobile wheel and tyre are suspended in the horizontal plane by a vertical steel rod 5mm in diameter and 2m long, which is bolted to the wheel axis. The wheel is given a small angular deflection, and makes 10 oscillations in 30.2s. Calculate the polar moment of intertia of the wheel and rod. Assume G = 80 GNm-2.

Homework Equations



I've got the following equations but I don't think I've got the right variables to be able to use them. Is there something I'm missing?

[tex]\theta=[/tex][tex]\frac{TL}{JG}[/tex]

Where [tex]\theta[/tex] = torsional deformation, T = torque, L = length, J = polar moment of inertia, G = shear modulus

Also

k = [tex]\frac{T}{\theta}[/tex]

I'm not sure where the number of oscillations comes into it, although I know period = 0.302s


Any help much appreciated, thanks
 
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  • #2
If you know the period, do you know how to find the frequency?

Assuming that you know how to find the frequency (in rad/s), then do you know how that is related to k and J?

You need to be careful here; there are two different J's in this problem. One is the mass moment of inertia of the wheel, the thing you are supposed to find, and the second is tha area moment of inertia of the rod. Pay attention to which is which. It would be a good idea to denote them Ja and Jw or some such just to keep yourself clear on this.
 
  • #3
a3sportback54: The two relevant equations you listed are correct. Notice you can substitute one of these equations into the other. As the second paragraph of the post by Dr.D points out, you need to list one more relevant equation. I would probably use the nomenclature I = wheel and tyre mass polar moment of inertia, and J = steel rod area polar moment of inertia. Also, period is not 0.302 s; try that one more time. Here are some questions to help you figure out the third relevant equation. What is the relation (equation) between period tn (s) and frequency f (Hz)? What is the relation between frequency f (Hz) and circular frequency omega (rad/s)? What is the relation between omega, k, and I?
 

FAQ: Calculating Polar Moment of Inertia for Automobile Wheel and Tyre

What is the Polar Moment of Inertia?

The Polar Moment of Inertia is a measure of an object's resistance to rotational motion around the axis perpendicular to its plane of motion.

How is the Polar Moment of Inertia calculated?

The Polar Moment of Inertia is calculated by summing the products of each particle's mass and its squared distance from the axis of rotation.

What is the difference between Polar Moment of Inertia and Moment of Inertia?

The Moment of Inertia is a measure of an object's resistance to rotational motion around any axis, while the Polar Moment of Inertia specifically measures resistance to rotation around an axis perpendicular to the plane of motion.

Why is the Polar Moment of Inertia important?

The Polar Moment of Inertia is important in engineering and physics as it helps determine an object's stability and ability to resist changes in rotational motion.

How does the shape of an object affect its Polar Moment of Inertia?

The shape of an object directly affects its Polar Moment of Inertia, as it determines the distribution of mass around the axis of rotation. Objects with more mass concentrated near the axis will have a smaller Polar Moment of Inertia, while objects with more mass spread out from the axis will have a larger Polar Moment of Inertia.

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