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--- ___ do you like dolphins?
---Because they are very smart.

A．What

B．When

C．Why

D．How

質量為60kg的小明站在水平地面上，利用如圖所示裝置提升物體A。物體A的質量m_A為72kg，底面積S_A為2×10^-2m²。當小明施加豎直向下的拉力為F₁時，物體A未被拉動，此時物體A對地面的壓強p為5×10³Pa，小明對地面的壓強為p₁；當小明施加豎直向下的拉力為F₂時，物體A恰好勻速上升，小明對地面的壓強為p₂，且p₁：p₂="16:" 15。已知B、C、D、E四個滑輪的質量相同，不計繩重和摩擦，g取10N/kg。求：

（1）當小明施加豎直向下的拉力F₁時，物體A對地面的壓力F_N；
（2）每個滑輪所受的重力G₀；
（3）當小明施加豎直向下的拉力F₂時，整個裝置的機械效率η。

C
（1）依據公式p=

，可知F_N=pS=5×10³Pa×2×10^-2m²="100N " ---------- 2分

（2）當小明施加豎直向下的拉力F₁時，物體A、滑輪B、滑輪C及小明的受力情況如答圖7所示。

T_A1=G_A- F_N′=720N-100N=620N
F₁=

小明對地面的壓強p₁=

當小明施加豎直向下的拉力F₂時，物體A、滑輪B、滑輪C及小明的受力情況如答圖8所示。

F₂=

小明對地面的壓強p₂=

動滑輪受到的重力G₀="60N " ---------- 3分
（3）拉力F₂=

=225N
設物體A被提升的高度為h，則滑輪C上升的高度h_C=2h，
繩端上升的距離s=2h_C=4h
η=

---------- 2分解析:
由because引導的答語，其問句要用why來詢問。略

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