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PS What's a good explanation for why a wheel rolls downhill, in terms of forces?

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- Thread starter ellerbro
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PS What's a good explanation for why a wheel rolls downhill, in terms of forces?

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ideasrule

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PS What's a good explanation for why a wheel rolls downhill, in terms of forces?

I think the most important reason for the difference

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russ_watters

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But there are other forms of resistance that are probably not directly proportional to weight either. The rolling friction of the tires will vary with weight, but probably not linearly and viscous friction in the bearings of the wheels probably won't vary that much with the weight of the rider. In addition, the moment of inertia of the wheels (resistance to angular acceleration) will not be affected by the weight of the rider.

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russ_watters

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The force is due to his mass, but he resists acceleration due to his mass, so it cancels out. This is why all objects, when dropped, fall at the same speed regardless of their mass.

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rcgldr

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What doesn't change is the angular kinetic energy of the wheels, which reduces the rate of acceleration. If all of the mass of the bicycle were in the rims and tires, then the rate of acceleration down the inclined plane with the tires rolling would be 1/2 that if the inclined plane were frictionless and no angular energy was added to the wheels. As the ratio of mass of the bicycle and rider increases compared to the weight of the rolling rims and tires, the smaller the overall impact of the angular energy in the wheels versus the linear kinetic energy of the bicycle and rider, and the faster the rate of acceleration. This is ingoring factors such as the tires would deform more under a heavier load, which would create more rolling resistance.

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The force is due to his mass, but he resists acceleration due to his mass, so it cancels out.

That definitely clears things up. Thanks for all your input folks.

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rcgldr

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Except that mass in the rims of a bicycle accelerate at half the rate of the non-rotating mass. The higher the ratio of non-rotating mass to rotating mass, the faster the rate of acceleration.

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