如图所示,物体在F1、F2作用下在水平地面上保持静止。若已知F1=15 N,F2=8 N,物体所受的合力大小是____ N,地面对物体的 摩擦力的大小是____ N。
如图,∠B+∠C+∠D+∠E﹣∠A等于( )
China is greatly developing the energy from the sun and wind. These new types of energy cost very little and will never ____.