Inertia with the parallel axis theorem

In summary, for a uniform solid sphere with a moment of inertia I about an axis tangent to its surface, the moment of inertia about an axis through its center is equal to I - (2/5)MR^2.
  • #1
Northbysouth
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2

Homework Statement


A uniform solid sphere has a moment of inertia I about an axis tangent to its surface. What is the moment of inertia of this sphere about an axis through its center?


Homework Equations



Ip = Icm + Md2

Isolid sphere = 2/5MR^2

The Attempt at a Solution



This is what I tried:

Ip = 2/5MR^2 + MR^2
=7/5MR^2

I'm not sure what I'm supposed to do with this. I know that the correct answer is 2/7 but I don't know how you get that answer.
 
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  • #2
In this case you're told that Ip = "I", where "I" is just meant to be some given value. You've already worked out from the parallel axis theorem that if the moment of inertia around the parallel axis is I, then the moment of inertia around the centre is I - Md^2, where d = r here.

You ended up with I = 7/5MR^2. If 7/5MR^2 = I, then what is Icm = 2/5MR^2 equal to in terms of the given value, I?
 

1. What is inertia with the parallel axis theorem?

Inertia with the parallel axis theorem is a concept in physics that describes the resistance of an object to change its state of motion. The parallel axis theorem states that the moment of inertia of an object is greater when it is rotated around an axis parallel to its center of mass, compared to when it is rotated around its center of mass.

2. How is the parallel axis theorem applied in physics?

The parallel axis theorem is applied in physics to calculate the moment of inertia of an object when it is rotated around an axis parallel to its center of mass. It is used to determine the distribution of mass in an object and how it affects its rotational motion.

3. What factors affect the moment of inertia of an object?

The moment of inertia of an object is affected by its mass and how the mass is distributed around its axis of rotation. The farther the mass is from the axis of rotation, the greater the moment of inertia will be. Therefore, the shape and size of an object can also affect its moment of inertia.

4. How is the parallel axis theorem different from the perpendicular axis theorem?

The parallel axis theorem and the perpendicular axis theorem are both used to calculate the moment of inertia of an object. However, the parallel axis theorem is used for objects that rotate around an axis parallel to its center of mass, while the perpendicular axis theorem is used for objects that rotate around an axis perpendicular to its center of mass.

5. Why is understanding inertia with the parallel axis theorem important?

Understanding inertia with the parallel axis theorem is important in many fields of physics, such as mechanics, dynamics, and kinematics. It allows us to predict and analyze the rotational motion of objects and understand how mass distribution affects an object's behavior. It is also essential in engineering and design, as it helps in creating stable and efficient structures and machines.

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