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This is a very common nonsense statement in elementary courses. Try picking up a pencil and moving it. Not too hard, is it? Now imagine trying it without friction. What force do they think set the pencil in motion? Put a chair in a pickup truck with the tailgate open. Now drive away. Without friction, the chair doesn't move with your truck and gets left back on the street after the truck slides out from under it.
It's not surprising that the claim that friction always opposes motion is wrong. It violates the basic principle that motion is relative. The same force that slows something down according to one observer speeds it up according to another.
(published on 04/12/11)
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