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Answers for “如何使用垂直条带设置关于 y 轴的面积惯性矩积分?”

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To set up the integral for IyI_y using a vertical strip, first express the differential area dAdA in terms of xx. A vertical strip has an infinitesimal width dxdx and a height determined by the difference between the upper and lower boundary curves at position xx.

Conditions: The region is bounded by curves that can be expressed as functions of xx.; A vertical differential strip is chosen, meaning its width is dxdx.; The integration variable is xx.

Find a method

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To use the curve equation y2=2xy^2 = 2x in an integral setup with vertical strips (where the height must be a function of xx), you must solve for yy. Taking the square root of both sides gives y=±2xy = \pm\sqrt{2x}.

Conditions: The boundary curve is given as y2=2xy^2 = 2x.; The region is in the first quadrant (x≥0,y≥0x \ge 0, y \ge 0).; Vertical strips are being used, requiring height as a function of xx.

Understand why

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The area moment of inertia about a specific axis is defined by integrating the square of the perpendicular distance from that axis over the entire area. For the y-axis, the perpendicular distance from any differential area element to the axis is the horizontal coordinate xx.

Conditions: The moment is computed over a planar area in the xy-plane.; The axis of interest is either the x-axis or the y-axis.; The distance is measured perpendicularly from the axis to the differential area element.

Meet the concept

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The final numerical value of the area moment of inertia IyI_y for the specific shaded region about the centroidal y-axis is approximately 0.762 m40.762 \text{ m}^4. This value is obtained by evaluating the definite integral ∫02x2(2−2x1/2)dx\int_0^2 x^2 (2 - \sqrt{2} x^{1/2}) dx, which simplifies to 243−2(27)23.5\frac{2^4}{3} - \sqrt{2} \left(\frac{2}{7}\right) 2^{3.5}.

Conditions: The region is bounded by y=2y = 2 (top) and y=2x1/2y = \sqrt{2} x^{1/2} (bottom) from x=0x=0 to x=2x=2.; The axis is the centroidal y-axis.; Units are in meters.

Know when to use it

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You can skip the parallel axis theorem when the problem asks for the area moment of inertia directly about one of the coordinate axes used in the integral setup, provided there is no perpendicular offset distance to account for. In the specific example shown, the goal is to find IyI_y about the y-axis itself.

Conditions: The requested axis is one of the coordinate axes (e.g., the x-axis or y-axis).; The integral is set up directly about that same axis.; No separate shifted-axis correction is being applied in the setup.

Understand why

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The units of the area moment of inertia are derived directly from its defining integral, I=∫r2dAI = \int r^2 dA. The differential area element dAdA has units of length squared (e.g., m2m^2).

Conditions: Lengths are measured in meters in the specific example.; The integral represents an area moment of inertia.