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Coefficient of friction and dynamic and static friction

It's not a problem but I need a good explanation please because I really don't understand:

I need someone to explain why when we have (for example) one block on another, why when we represent the friction force, its direction can sometimes become, in the same sense, not only direction but also acceleration? How is this possible? Friction is always in the opposite sense of acceleration!

Could you use drawings please and explain at the same time why we have static or dynamic friction?
Thanks.

Solution Preview

Assume a block is sitting on a table and we pull it with string with an applied force of Fa Newtons. The friction always acts to oppose the motion of the block and hence is pushing the block in the opposite direction. The frictional force is given by Ff = (Mu)*N, where N is the Normal force which is perpendicular to the motion and hence, in this case, equal to the weight of the block, W Newtons and "Mu" is the coefficient of friction.

There ...

Solution Summary

This solution is provided in 310 words. It explains force and friction, as well as defines static and dynamic friction. It also discusses how acceleration is related to friction and provides references to further learning.

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