Quote:
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Originally Posted by mylesdw
Go on then, put me out of my misery! What have I done wrong:
We have a 40 lb flywheel 11" in diameter at 800 rpm,
Rotational mass (I)= 0.5 * m * r^2
so
I = 2420 = 0.5 * 40 * 11^2
Energy = 0.5 * I * w^2
so
774400000 = 0.5 * 5280 * 800^2
We also have a 2800 lb car travelling at 5 mph,
to make a comparison we need to convert mph to inches per minute
v = 5 / 60 * 63359 = 5280
Energy = 0.5 * m * v^2
39028528010 = 0.5 * 2800 * 5280^2
My energies are a factor of 50 apart!
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Units,units, units...
Your inertia is in lb*in². It is 16.8 lb*ft².
However, your rotational speed should be in radians/second not revolutions/min. That is a factor of 377.
So you energy becomes 1/2 * 16.8 lb*ft² * (800/(60*2*pi) rad/sec)². This equals 1800 lbf-ft.
At 5mph your car is traveling 7.33 ft/sec. So your equation is 1/2 * 2800 lb * (7.33 ft/sec)² which equals 2340 lbf*ft.
The 40# figure is a little light. 45-50# is probably closer.