How do you find the area of this? I had to do an experiment where you take a styrofoam cup and poke a flashlight through the bottom of it, then shine the light onto a wall from different distances and measure the diameter. I have to calculate the area. It it just pi*r^2? That's what I did, but then I've seen someone else use 4*pi*r^2 and that confused me. I want to make sure I done it right.

How do you find the area of this? I had to do an experiment where you take a styrofoam cup and poke a flashlight through the bottom of it, then shine the light onto a wall from different distances and measure the diameter. I have to calculate the area. It it just pir^2? That's what I did, but then I've seen someone else use 4pi*r^2 and that confused me. I want to make sure I done it right.

University Physics Volume 1

18th Edition

ISBN:9781938168277

Author:William Moebs, Samuel J. Ling, Jeff Sanny

Publisher:William Moebs, Samuel J. Ling, Jeff Sanny

Chapter1: Units And Measurement

Section: Chapter Questions

Problem 62P: (a) Estimate the density of the Moon. (b) Estimate the diameter of the Moon. (c) Given that the Moon...

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That did answer a lot of questions. Thank you. But where do you derive the equation E=(K|q|)/r? My book says E=(K|q|)/r^2. I tried re-arranging some equations to get your equation, but I can't figure it out. Thanks.
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