g = mg = 100kg× 10N/kg = 1000n。

(2) When the wooden box is placed on the horizontal ground, the pressure on the horizontal ground is:

F=G= 1000N，

Pressure on level ground:

p = FS = 1000n 0.2m 2 = 5000 pa；

(The horizontal thrust of 300N can make the wooden box move at a uniform speed, that is to say, the friction and thrust on the wooden box are a pair of balanced forces in the horizontal direction, and the friction is equal to the thrust, which is equal to 300 N. 。

So the answer is:

( 1) 1000n；

(2)5000 Pa;

(3)300N。