Rotational Inertia of a triangle

In summary, the question asks for the rotational inertia of a thin, uniform vane in the shape of a right triangle, with a vertical axis passing through its apex. The correct answer in terms of the triangle's base width and mass is ##b^2M/2##, but it is unclear if the axis is perpendicular to the plane of the triangle or in the plane of the triangle. The word "vane" suggests that the axis is coplanar with the vane.
  • #1
rpthomps
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Homework Statement



A thin, uniform vane of mass M is in the shape of a right triangle, as shown. Find the rotational inertia about a vertical axis through its apex, as shown in the figure. Express your answer in terms of the triangle’s base width b and its mass M.

Homework Equations

The Attempt at a Solution



Question.jpg


The answer in the back is just ##b^2M/2## without the second term.
 
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  • #2
It's not clear what is meant by "a vertical axis through the apex". Your answer looks correct if the vertical axis is perpendicular to the plane of the triangle and passes through the lower left vertex of the triangle in your drawing. The answer in the back of the text appears to be correct if the axis is in the plane of the triangle, passes through the lower left vertex, and is perpendicular to the base b.
 
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Likes rpthomps
  • #3
Okay, thanks for looking it over.
 
  • #4
The word 'vane' implies to me that the axis is coplanar with the lamina.
rpthomps, your r is wrong. You want the distance from the axis, not the distance from the apex.
 

What is rotational inertia?

Rotational inertia, also known as moment of inertia, is a measure of an object's resistance to changes in its rotational motion. It depends on the object's mass, shape, and distribution of mass.

How is rotational inertia calculated for a triangle?

The rotational inertia of a triangle can be calculated using the equation I = (1/6) x m x a^2, where I is the moment of inertia, m is the mass of the triangle, and a is the length of one of its sides.

What is the difference between rotational inertia and moment of inertia?

Rotational inertia and moment of inertia are two terms that are often used interchangeably. Both refer to the measure of an object's resistance to changes in its rotational motion.

How does the shape of a triangle affect its rotational inertia?

The shape of a triangle can greatly affect its rotational inertia. A triangle with a larger base and shorter height will have a higher rotational inertia compared to a triangle with a smaller base and longer height. This is because the mass is distributed further from the axis of rotation in the first triangle, making it more difficult to rotate.

What are some real-life applications of understanding rotational inertia of a triangle?

Understanding the rotational inertia of a triangle can be useful in engineering and design applications, such as building structures, bridges, and machines. It can also be applied in sports, such as gymnastics and figure skating, where understanding the distribution of mass in a body can help improve performance and prevent injury.

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