If the gravity of the eraser is known to be 1.5n, the attraction between the eraser and the blackboard is constant 4N. When a thrust of 0.5N is applied to the eraser, the eraser can slide along the blackboard surface at a constant speed. If the eraser is to slide along the blackboard surface at a constant speed, a vertical upward force should be applied. The force is 0.5N___ ___ N．

If the gravity of the eraser is known to be 1.5n, the attraction between the eraser and the blackboard is constant 4N. When a thrust of 0.5N is applied to the eraser, the eraser can slide along the blackboard surface at a constant speed. If the eraser is to slide along the blackboard surface at a constant speed, a vertical upward force should be applied. The force is 0.5N___ ___ N．

In the horizontal direction, the attraction of the blackboard to the eraser and the supporting force of the blackboard to the eraser are a pair of balanced forces. In the vertical direction, the eraser is subject to the vertical downward gravity, the vertical downward thrust and the vertical upward friction. Therefore, the friction f = G + F = 1.5n + 0.5N =