Drexel dragonThe Math ForumDonate to the Math Forum

Ask Dr. Math - Questions and Answers from our Archives
_____________________________________________
Associated Topics || Dr. Math Home || Search Dr. Math
_____________________________________________

Diameter of a Ball


Date: 11/25/2001 at 22:55:18
From: Tabitha Howard
Subject: Physics

A child rolls a ball on a level floor 4.5m to another child. If the 
ball makes 15.0 revolutions, what is its diameter?

I don't know where to start.


Date: 11/26/2001 at 10:28:37
From: Doctor Ian
Subject: Re: Physics

Hi Tabitha,

Imagine that you paint a blue line around the widest part of the ball, 
and put a red dot at one point on this line. Then you roll the ball 
across the floor. You would see a pattern like this:

   -------*-------*---...---*-------*

where each '*' shows where the dot hit the floor, and each '-' shows 
where the blue line was hitting the floor. (We rolled the ball while 
the paint was still wet!)

Now, there will be one '*' for each time that the ball makes a 
revolution. So if the ball makes 15 revolutions, we'll see this:

  ---*---*---*---*---*---*---*---*---*---*---*---*---*---*---*

Each '---*' is the length of the blue line, which is to say, each 
'---*' is one circumference of the ball. So here's the situation:

  ---*---*---*---*---*---*---*---*---*---*---*---*---*---*---*
   c   c   c   c   c   c   c   c   c   c   c   c   c   c   c

  |----------------------------------------------------------|
                     4.5 meters

From this, we can find out what the circumference must be.  (15 
circumferences add up to 4.5 meters.) And from the circumference, we 
can find the diameter, using the formula

  circumference = pi * diameter

Can you take it from here?

- Doctor Ian, The Math Forum
  http://mathforum.org/dr.math/   
    
Associated Topics:
High School Conic Sections/Circles
High School Geometry
High School Higher-Dimensional Geometry

Search the Dr. Math Library:


Find items containing (put spaces between keywords):
 
Click only once for faster results:

[ Choose "whole words" when searching for a word like age.]

all keywords, in any order at least one, that exact phrase
parts of words whole words

Submit your own question to Dr. Math

[Privacy Policy] [Terms of Use]

_____________________________________
Math Forum Home || Math Library || Quick Reference || Math Forum Search
_____________________________________

Ask Dr. MathTM
© 1994-2013 The Math Forum
http://mathforum.org/dr.math/