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Special topic: computational problems; Work, power and mechanical efficiency.

Analysis: It is known that the mass of object A is 4kg, and the gravity of object A can be calculated according to G=mg, and then the work done by object A can be calculated according to W=Gh; Then calculate the power according to P=.

Gravity of object: G=mg=3kg× 10N/kg=30N,

The work done by object A is: W=Gh=30N×0.4m= 12J,

Power: p = = 10W.

So the answer is: 10.

Comments: It is not difficult to check the calculation of work and power, so it can be solved directly by formula. Pay attention to finding the gravity of an object by mass.