Statics 5The way to deal with sign of inequality in problems with friction NF. But in general, we only consider the conditions when the motion is impending(.e, NfF), and then append a sign of inequality on the result or judge the range of equilibrium. Therefore we can avoid troubles and miscalculations which would occur in solving inequations 2. Basic equations and theories (1)Basic theories(in common use) O Three coplanar equivalent forces must intersect at one point (to determine the direction of the unknown forces 2 The law of projection of the resultant force: REX The law of the moment of a resultant force: mo(R)=> m o(F7) projection equation m(r)=2m(Fi)9 ⑤The way to deal with sign of inequality in problems with friction: ∵Nf≥F. But in general, we only consider the conditions when the motion is impending (i.e., Nf=F ), and then append a sign of inequality on the result or judge the range of equilibrium. Therefore we can avoid troubles and miscalculations which would occur in solving inequations. (1) Basic theories (in common use) ① Three coplanar equivalent forces must intersect at one point. (to determine the direction of the unknown forces) ② The law of projection of the resultant force: RX =X ③ The law of the moment of a resultant force: projection equation: ( )= ( ) mO R mO Fi ( )= ( ) mz R mz Fi 2. Basic equations and theories: