Suppose there is a block on an inclined plane.A horizontal force is being applied on the block, making it stay at rest.What is the direction of friction in this case and why?
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Friction is always parallel to the surface, so you start from there. Since the block is static, the horizontal force will be opposed by a combination of the friction and the normal to the surface. For example, if you deconstruct the horizontal force into two vectors, one perpendicular to the surface and one parallel, the perpendicular force will be opposed by the surface itself (much like the ground opposes gravity while you stand) and the parallel force will be opposed by the friction.