Diameter of a BallDate: 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/ |
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