23 Nov 2019

A person is pulling a block of mass m w/ a force equal to itsweight directed 30 degrees above the horizontal plane across arough surface generating a friction f on the block. If the personis now pushing downward on the block with the same force 30 degreesabove the horizontal plane across the same rough surface what isthe friction on the block? Answer is 3f

Explanation states F=ukN where N is normal force acting on block. Iunderstand that however it then states when force is applied 30degrees above horizontal N=mg-mgsin30? How is that the normal forceI thought the normal was mgcos30? It the states when force isapplied 30 degrees below horizontal N=mg+mgsin30? Solving for N andplugging it into F=ukN and solving for uk gives 3f.

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