A $50$ cm x $30$ cm x $20$ cm block weighing $150$ N should be moved at a constant speed of $0\mathrm{.}8$ m$/$s up an

oblique plan with a coefficient of friction f$=0\mathrm{.}27.$

a$)$ Determine the force F required in the horizontal direction. Hint: friction force$=$ f $$ normal force.

b$)$ Determine the percentage of force reduction if there is a $0\mathrm{.}4$ mm thick oil film between the block

and the oblique plane. The oil has a dynamic viscosity of $0\mathrm{.}012$ Pa $$ s$.$ Solution: F $=105\mathrm{.}5$N$,45\mathrm{.}8\%$ reduksjon.