# eli5 Tire rolling down hill

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I am literally 5 compared physicists-a tire rolls down hill and it keeps rolling faster can it increase in speed infinitely as long as the hill is infinitely large?

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In a vacuum? Possibly. Not in a vacuum no it’ll hit terminal velocity at some point. Do to air friction , water friction , and friction with the hill and possibly other forces it will have to max out eventually. In a vacuum with no air or water it would most likely speed up close to infinite I suppose

In classical physics, yes.

BUT, under relativity, the closer the ball gets to the speed of light, the more energy it needs to “push” it the same amount.

So it would essentially get closer and closer to the speed of light.

*this is assuming we ignore wind resistance, and the fact that the tire would tear itself apart well before anything relativistic starts to take effect.*

No, a tire would not be able to accelerate infinitely. Even in a vacuum, the sheer centrifugal force would pull the tire apart.

As it approaches the speed of light, it would take an ever-increasing amount of force to pull it faster. No physical object can reach the speed of light, and nothing including light can exceed it. So could it *increase* in speed forever? Yes but the rate it accelerates will slow so it inches ever closer to the speed of light but never reaches it.

In the real world the “speed limit” would be terminal velocity where air resistance is slowing it as much as gravity is speeding it up. The behavior is the same, it will slowly approach this speed limit but won’t pass it.

Setting aside relativistic limitations (the increasing amount of energy it takes to accelerate mass as it reaches the speed of light), you cannot place an object at an “infinite” height atop an infinite hill.

It doesn’t make sense in a real physical sense to say two objects are infinitely far apart. Two physical items cannot be literally infinitely far apart. As long as you’re talking about two real objects that exist somewhere in space, there is a finite amount of space separating them.