g = mg = 20k g× 10N/kg = 200n；

(2) Speed of counterweight movement:

v = ST = 0.4 m2s = 0.2m/s；

(3) Total work done by tension:

W total =Fs=250N×0.4m= 100J，

Pull to do useful work:

W has =Gh=200N×0.4m=80J，

Mechanical efficiency of tensioner;

η=W, where w totals ×100% = 80100×100% = 80%.

Answer: (1) The gravity of the pulled counterweight is 200 N;

(2) The speed of the counterweight in 2S is 0.2m/s;

(3) The mechanical efficiency of the tensioner in this process is 80%.