∴ tension f = ghη s = 900 n×1m 90 %× 5m = 200 n;

(2)√η=W Useful w Total = W Useful Fs,

∴W has =ηW total =ηFs,

∫W Total =W Useful +W Quantity

∴W quantity =W total -W useful =Fs-ηFs=( 1-η)Fs=fs,

∴ Friction on an object: f = f (1-η).

Answer: (1) The pulling force is 200 N;

(2) The derivation process is as above.