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Old 17th December 2006, 13:40   #46
jat
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A/ Heavier car theory
If M is the mass of the car and it is pushed to a speed of v, then KE attained is .M.v2

And this energy is lost as work done against friction. That is if f is effective coeff of friction then work done = f.M.g.s

Means .M.v2 = f.M.g.s
Or s = .v2/f/g


B/ Heavier Log therory
We consider a thin shell hollow cylinder made of lighter material such as aluminium and another one of heavier material such as iron. Assume the coeff of friction is same.

Then energy attained at speed v = .I.w2 = .m(alum).r2.w2 and .m(iron).r2.w2

And work done against friction is f.m.g.s where s = rotational distance traveled.
Therefore
.m(alum).r2.w2 and .m(iron).r2.w2 = f.m(alum or iron).g.s
or s = .r2.w2/f/g

Therefore the distance covered is entirely free of mass of the car of log when it is allowed to stop due to without any external aid.

C/ Heavier flywheel

If T is the net mean effective torque produced by the engine before flywheel and I is moment of inertia of flywheel and f is coeff of friction then torque available to accelerate the car after the flywheel is

T = T- .I.a f.m.g.k
where k is effective radius at which the friction due to weight of flywheel is assumed to be working (bearing friction) and a = angular acceleration

From this we can see that if we increase the mass of flywheel, 'I' and 'm' both are going to increase and hence net mean effective torque available is going to reduce.

Therefore putting a heavier flywheel is going to reduce the effective torque and not increase the torque.
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Old 17th December 2006, 14:28   #47
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Agreed.

But, i don't get how torque at the flywheel could reduce? If T = F * r, then the amount of torque being applied to turn the flywheel would be the same. However, the power being produced (at the flywheel) would reduce, as the revolutions of the heavier flywheel would be lesser compared to that of a light one.

It's like your ceiling fan. Whether you use light blades or heavy ones, the turning force applied on them would be the same. But the lighter blades would spin a lot quicker/faster than the heavier ones.

Shan2nu

Last edited by Shan2nu : 17th December 2006 at 14:41.
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Old 17th December 2006, 14:33   #48
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You are absolutely right. The torque at crank isn't affected at all by the weight of the flywheel.
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