W=FS=250N×0.4m= 100J，

Power of pulling force: p = wt =100j2s = 50w;

(2)W total =W= 100J, useful:

W useful =ηW total =80%× 100J=80J,

Weight of counterweight:

G=W useful h = 80J0.4m = 200N

Answer: (1) When Xiaoming pulls the pole, the pulling power is 50W；;

(2) The weight of the pulled counterweight is 200 N;

(3) Because there is no moving pulley in this tensioner, it can't save labor, but it is laborious because of the existence of friction;

Because of the crown block, the direction of force can be changed. The direction of force can be changed by lifting the weight up by the crown block, and the weight can be lifted by pulling down the rope.