Cross-Sections, Rotations of 3D Objects

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Geometry › Cross-Sections, Rotations of 3D Objects

Questions 1 - 10
1

A right circular cylinder stands upright. A plane slices the cylinder perpendicular to the circular bases and passes through the cylinder’s central axis (so the plane contains the axis). Which description correctly identifies the resulting shape?

Trapezoid

Rectangle

Ellipse

Circle

Explanation

This problem involves finding the cross-section of a 3D solid. The original solid is a right circular cylinder standing upright with circular bases. The slicing plane is perpendicular to the circular bases and passes through the central axis. As the plane cuts through, it follows the height and spans the diameter of the bases. The resulting cross-section is a rectangle with sides equal to the height and diameter. A common misconception is thinking it forms a circle, but that's for a parallel slice. To visualize, imagine unfolding the cylinder along the axis step by step to see the rectangular outline.

2

A rectangular prism has dimensions $6$ cm (length), $4$ cm (width), and $5$ cm (height). A plane slices the prism parallel to the base (the $6\times 4$ face). Which shape results from the cross-section shown?

Circle

Rectangle

Triangle

Pentagon

Explanation

This problem involves finding the cross-section of a 3D solid. The original solid is a rectangular prism with rectangular faces. The slicing plane is parallel to the base, which is the rectangular face. As the plane cuts through the prism, it intersects the lateral faces evenly. The resulting cross-section is a rectangle identical to the base. A common misconception is assuming it forms a triangle, which isn't possible with a parallel slice. To visualize, imagine layering the prism and slicing parallel to the bottom step by step to match the base shape.

3

A right circular cylinder has radius $4$ cm and height $10$ cm. A plane slices the cylinder parallel to the circular bases (so the plane is horizontal). Which shape results from the cross-section shown?

Ellipse

Circle

Triangle

Rectangle

Explanation

This problem involves finding the cross-section of a 3D solid. The original solid is a right circular cylinder with circular bases. The slicing plane is parallel to the circular bases, making it horizontal. As the plane cuts through the cylinder, it intersects the curved surface evenly at every point. The resulting cross-section is a circle identical in shape to the bases. A common misconception is assuming it forms a rectangle, which occurs with a perpendicular slice instead. To visualize, imagine slicing a tube parallel to its ends step by step to reveal the circular shape.

4

A right circular cone has a circular base and a single apex. A plane slices the cone parallel to the base and does not pass through the apex. Which shape results from the cross-section shown?

Parabola

Circle

Rectangle

Triangle

Explanation

This problem involves finding the cross-section of a 3D solid. The original solid is a right circular cone with a circular base and a single apex. The slicing plane is parallel to the base and does not pass through the apex. As the plane cuts through the cone, it intersects the slanted sides uniformly. The resulting cross-section is a circle smaller than the base. A common misconception is assuming it forms a triangle, which happens when slicing through the apex. To visualize, imagine cutting a cone parallel to its base step by step to reveal the scaled-down circular shape.

5

A square pyramid has a square base and four triangular lateral faces meeting at an apex. A plane slices the pyramid parallel to the square base and below the apex. Which description correctly identifies the resulting shape?

Square

Triangle

Circle

Rectangle

Explanation

This problem involves finding the cross-section of a 3D solid. The original solid is a square pyramid with a square base and triangular faces meeting at an apex. The slicing plane is parallel to the square base and below the apex. As the plane cuts through, it intersects the triangular faces proportionally. The resulting cross-section is a square smaller than the base. A common misconception is thinking it forms a rectangle, but the square base ensures a square slice. To visualize, imagine truncating the pyramid parallel to the base step by step to reveal the scaled square.

6

A plane slices a right circular cylinder at an oblique angle: the plane is not parallel to the bases and not perpendicular to the bases. The plane intersects the curved surface and does not pass through an edge (the cylinder has no edges). Which shape results from the cross-section shown?

Rectangle

Ellipse

Circle

Triangle

Explanation

This problem involves finding the cross-section of a 3D solid. The original solid is a right circular cylinder. The slicing plane is at an oblique angle, neither parallel nor perpendicular to the bases. As the plane cuts through, it intersects the curved surface in a stretched manner. The resulting cross-section is an ellipse due to the angled intersection. A common misconception is assuming a circle, but the tilt elongates it into an ellipse. To visualize, imagine tilting the slice through the cylinder step by step to see the oval shape emerge.

7

A right circular cone is sliced by a plane that passes through the apex and also intersects the circular base. The plane contains the cone’s central axis. Which shape results from the cross-section shown?

Ellipse

Circle

Rectangle

Triangle

Explanation

This problem involves finding the cross-section of a 3D solid. The original solid is a right circular cone with a circular base and apex. The slicing plane passes through the apex, intersects the base, and contains the central axis. As the plane cuts through, it follows the axis from apex to base edges. The resulting cross-section is a triangle formed by the apex and base chord. A common misconception is thinking it forms a circle, but that's for a parallel slice away from the apex. To visualize, imagine splitting the cone along its axis step by step to expose the triangular profile.

8

Which solid is formed by rotating a right triangle about one of its legs (that leg is the axis of rotation)?

A sphere

A cylinder

A cone

A prism

Explanation

This problem examines solids of revolution created by rotating 2D shapes. The original shape is a right triangle with one 90-degree angle and two perpendicular legs. The axis of rotation is one of the legs (perpendicular sides), around which the triangle spins. As the right triangle rotates about one leg, the hypotenuse traces out a slanted circular surface while the other leg sweeps from the axis outward to create the base. The resulting solid is a cone with the rotating leg as its height and the other leg determining the base radius. Students might incorrectly choose cylinder, which comes from rotating rectangles, not triangles. To understand this, imagine spinning a right triangular flag on its pole—the flag creates a cone shape.

9

A square pyramid is sliced by a plane that is parallel to the base (the plane does not pass through the apex). Which description correctly identifies the resulting shape?

A rectangle

A triangle

A circle

A square

Explanation

This problem involves cross-sections of three-dimensional pyramids. The original solid is a square pyramid with a square base and triangular faces meeting at an apex. The slicing plane is parallel to the square base, cutting horizontally through the pyramid below the apex. When a plane parallel to the base cuts through a pyramid, it intersects all four triangular faces at the same proportional height, creating a shape similar to the base. The resulting cross-section is a square because the pyramid maintains its square symmetry at every horizontal level, just smaller as you go up. Students might incorrectly choose triangle, thinking all pyramid cross-sections are triangular. To understand this, imagine slicing a square-based pyramid cake horizontally—each layer reveals a square shape.

10

A cube is sliced by a plane that is parallel to one of its faces. Which shape results from the cross-section shown?

A triangle

A rectangle

A square

A circle

Explanation

This question examines cross-sections of three-dimensional solids, specifically cubes. The original solid is a cube with six square faces, all equal in size and perpendicular to adjacent faces. The slicing plane is parallel to one of the cube's faces, cutting through the cube without tilting. When a plane parallel to a face cuts through a cube, it intersects the four perpendicular faces along parallel lines of equal length. The resulting cross-section is a square identical to the cube's faces because the cube maintains constant square cross-sections at every level parallel to its faces. A common misconception is choosing rectangle, not recognizing that all cube faces and parallel cross-sections are squares. To visualize this, imagine slicing a block of cheese parallel to its face—you always get a square slice.

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