$解：(1)h=5×3\ \text {m}=15\ \text {m}$
$s=3h=3×15\ \text {m}=45\ \text {m}$
$t=\frac sv=\frac {{ 45 }\ \text {m}}{{ 0.3 }\ \text {m/s}}=150\ \text {s}$
$(2)W=Fs={ 360 }\ \text{N}×{ 45 }\ \text{m}={ 16200 }\ \text{J}$
$P=\frac {W}{t}=\frac {{ 16200 }\ \text{J}}{{ 150 }\ \text{s}}={ 108 }\ \text{W}$
$(3)W_有=Gh={ 900 }\ \text {N}×{ 15 }\ \text {m}={ 13500 }\ \text {J}$
$W_{额外}=W-W_有=16200\ \text {J}-13500\ \text {J}=2700\ \text {J}$
$(4)变小；因为物重不变即有用功保持不变，而额外功增加了.$
