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In the typical case of a smooth object (like a block) sliding down a ramp, once the gravitational force down the ramp exceeds the level of static friction between the ramp and the block, it begins to accelerate down the ramp. The acceleration is mostly constant until the block gets to the bottom of the ramp (or incurs some kind of parasitic drag from the air).

A sticky item, such as a block made out of rubber, slides slowly down the ramp but doesn't accelerate. It appears to quickly hit its max velocity and then maintains that velocity until it gets to the bottom.

Why is that the case?

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    $\begingroup$ It might be accelerating but too slowly for our eyes to notice. $\endgroup$
    – Yashas
    Commented Feb 22, 2017 at 6:24
  • $\begingroup$ Do you have any evidence that the rubber block behaves in the way that you describe? I appreciate that you may be asking about a personal observation, but how reliable is this observation? Have you done experiments to repeat the result? Have you done any research and found any confirmation? Coefficients of sliding friction are quoted for rubber on various surfaces (eg en.wikibooks.org/wiki/Physics_Study_Guide/…), which suggests that rubber behaves like other solid materials. $\endgroup$
    – sammy gerbil
    Commented Feb 22, 2017 at 20:44

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I am not very sure about this but the downward acceleration decreases with increase in the coefficient of kinetic friction. Now here is the key idea: the coefficient need not be constant throughout the block's motion. I think that this occurs for all objects but is most noticable in objects like rubber. The frictional force is strongly dependent on the nature of the surfaces in question and also the force of interaction between them. The proposition is that as the block slides, its surface is changed or the nature of its interactions change such that it shows a departure from ideal fixed friction behaviour. This could be really strong in substances like rubber, etc. However i do not have any solid math proof or reall any experimental evidence so it is a pretty slim answer.

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