На книгу действуют: сила нормальной реакции опоры (N) и сила тяжести (mg). Причем, N=mg. Следовательно, если модуль вектороа N=mg, и эти вектора противоположно направлены, то по второму закону Ньютона тело имеет нулевое ускорение, следовательно оно покоится.
Зачем вам по столбиками. здесь просто нужно составить предложения: boys often run around in the classroom during the break. we often learn new words at the lesson girls sometimes draw funny pictures on the blackboard during the break boys usually talk loudly during the break we sometimes run and jump between the desks during the break we sometimes discuss different problems during the break teachers usually ask a lot of questions at the lesson teachers sometimes have a cup of tea or coffee during the break i sometimes read newspapers and wall newspapers during the break
