---  ___  do you like dolphins?
---Because they are very smart.
A．WhatB．WhenC．WhyD．How

質(zhì)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(zhì)量mA為72kg，底面積SA為2×10-2m2。當(dāng)小明施加豎直向下的拉力為F1時(shí)，物體A未被拉動(dòng)，此時(shí)物體A對(duì)地面的壓強(qiáng)p為5×103Pa，小明對(duì)地面的壓強(qiáng)為p1；當(dāng)小明施加豎直向下的拉力為F2時(shí)，物體A恰好勻速上升，小明對(duì)地面的壓強(qiáng)為p2，且p1：p2="16:" 15。已知B、C、D、E四個(gè)滑輪的質(zhì)量相同，不計(jì)繩重和摩擦，g取10N/kg。求：

（1）當(dāng)小明施加豎直向下的拉力F1時(shí)，物體A對(duì)地面的壓力FN；
（2）每個(gè)滑輪所受的重力G0；
（3）當(dāng)小明施加豎直向下的拉力F2時(shí)，整個(gè)裝置的機(jī)械效率η。
C
（1）依據(jù)公式p=，可知FN=pS=5×103Pa×2×10-2m2="100N "         ---------- 2分
（2）當(dāng)小明施加豎直向下的拉力F1時(shí)，物體A、滑輪B、滑輪C及小明的受力情況如答圖7所示。
 
TA1=GA- FN′=720N-100N=620N
F1=
小明對(duì)地面的壓強(qiáng)p1=
當(dāng)小明施加豎直向下的拉力F2時(shí)，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(duì)地面的壓強(qiáng)p2=

動(dòng)滑輪受到的重力G0="60N                                     " ---------- 3分
（3）拉力F2==225N
設(shè)物體A被提升的高度為h，則滑輪C上升的高度hC=2h，
繩端上升的距離s=2hC=4h
η=                        ---------- 2分解析:
由because引導(dǎo)的答語(yǔ)，其問(wèn)句要用why來(lái)詢問(wèn)。略
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