(1) When the object A moves in a straight line at a uniform speed in the horizontal plane, the pulling force f and friction force f of the moving pulley on the object A are a pair of balanced forces.

Therefore, if F pulls =f=20N, the pulling force of object A on the moving pulley is f' = 20n.

According to the characteristics of the moving pulley, the rope end tension f can be obtained as follows: f =12f' =12× 20n =10n;

(2) As shown in the figure, when the object is pulled by a moving pulley, the free end of the rope moves twice the distance with half the force.

Therefore, the moving speed of the spring dynamometer is v=2vA=2×0.4m/s=0.8m/s,

The moving distance of the end point of 5s pulling force is s=vt=0.8m/s×5s=4m,

Tensile work w = fs = 10n× 4m = 40J.

③ Tension power p = WT = 40J5s = 8W.

So the answer is:10; 40; 8.