A block of mass m is slipping down a fixed rough inclined plane of inclination α with constant speed. The contact force between block and incline is?

Ans : mg

Please explain the basic definition of contact force and how is it different from normal reaction.

Dear Nehal

Contact force has two component ,one is along the plane which is calledfrinction force and onther is alonh tthe normal to the plan which is called normal force.

in this case block is slipping with constant speed so mgsinα = μmgcosα ..........1

  and normal force = mgcosα  .................2

so resultant force  = √(μmgcosα)² + (mgcosα)²

                                   =√(mgsinα)² + (mgcosα)²

                                   =mg

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