Problems Plus 8: Volume of a Solid from its Shadows

in hive-196037 •  last month 


In this video I obtain the volume of the largest solid whose shadows are a square of height 1, a circle, and an isosceles triangle. I first start off with a circular cylinder and then cut off the sides to form a triangle when viewed from the side. I calculate the volume by taking thin vertical slices and summing up using an integral. The volume turns out to be about 0.45. Lastly, I show that the volume can be made arbitrarily small as long as the skeleton is maintained.

Timestamps:

  • Problem 8: Volume of a solid from its shadows: 0:00
    • Review of Exercise 44: Solid with square, isosceles triangle, and circle shadows: 0:44
  • Solution to (a): Volume of the largest such solid: 1:51
  • Start with a circular cylinder and shave off excess: 3:05
  • Cut solid with planes a parallel to the x-axis and form a triangle: 6:02
  • Compute volume of the solid via vertical slices parallel to xz plane: 8:21
    • Base of the solid is a circle with radius 0.5: 9:15
    • Obtain height from a triangle: 13:57
    • Determine cross section area: 15:54
  • Solve for the volume by taking an integral: 18:51
    • First integral is a quarter-circle: 21:10
    • Second integral is solved by substitution: 22:38
  • Volume is approximately 0.45: 28:50
    • Calculation check using an integral calculator: 30:39
  • Solution to (b): There is no smallest volume: 32:56
    • Can have arbitrarily small volume as long as the skeleton is still in tact: 34:33

Video notes and playlists:


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