The answer is 3 ft
The surface area (SA) of a cylinder with radius r and height h is:
SA = 2πr² + 2πrh = π(2r² + 2rh)
We have:
SA = 126π ft²
h = 3 * d
d = 2r
h = 3 * 2r = 6r
126π = π(2r² + 2rh)
126 = 2r² + 2rh
126 = 2r² + 2r * 6r
126 = 2r² + 12r²
126 = 14r²
r² = 126/14
r² = 9
r = √9
r = 3
You figure out how long it would take a car traveling at 25 mph
to cover 360 ft. Any driver who does it in less time is speeding.
(25 mi/hr) · (5,280 ft/mile) · (1 hr / 3,600 sec)
= (25 · 5280 / 3600) ft/sec = (36 and 2/3) feet per second.
To cover 360 ft at 25 mph, it would take
360 ft / (36 and 2/3 ft/sec) = 9.82 seconds .
Anybody who covers the 360 feet in less than 9.82 seconds
is moving faster than 25 mph.
_________________________________
If you're interested, here's how to do it in the other direction:
Let's say a car covers the 360 feet in ' S ' seconds.
What's the speed of the car ?
(360 ft / S sec) · (1 mile / 5280 feet) · (3600 sec/hour)
= (360 · 3600) / (S · 5280) mile/hour
= 245.5 / S miles per hour .
The teacher timed one car crossing both strips in 7.0 seconds.
How fast was that car traveling ?
245.5 / 7.0 = 35.1 miles per hour
Another teacher timed another car that took 9.82 seconds to cross
both strips. How fast was this car traveling ?
245.5 / 9.82 = 25 miles per hour
