1.An egg, initially at rest, is dropped onto a concrete
surface and breaks.With the egg treated as the system,
what is the sign of work?
(Given:△U + △E_p + △E_k = Q - W)
A.positive,done by the egg B.positive,done on the egg
C.No work (equal to zero) D.negative,done by the egg
E.negative,done on the egg
這題我先當他是自由落下,所以再碰地前是能量守恆,然後看起來
也沒有Q的流動,所以我覺得就只是看break算是正功或負功,蛋被
打破了,所以應該是被作功,我就猜答案(E) (我是猜E,原本打錯)
不過後來越想越奇怪阿@@而且最後系統還破掉,想問板上高手怎麼看
這題?
2.A 300 kg mass on a 25-m long string oscillates as a pendulum.
It has a speed of 14.14 m/s as it pass through the lowest point.
What maximum angle does the pendulum reach? [g=10 m/s^2 ]
這題我用最低點的動能 = 最高點的位能(以最低點U=0)來算得到53.13度
但答案沒這選項,雖然覺得這作法沒錯,但請各位喵一眼XD說不定題目
暗藏玄機?
一直問問題 先謝謝各位了XD
--
※ 發信站: 批踢踢實業坊(ptt.cc)
◆ From: 111.184.227.135
推
11/03 00:18, , 1F
11/03 00:18, 1F
→
11/03 00:37, , 2F
11/03 00:37, 2F
→
11/03 00:59, , 3F
11/03 00:59, 3F
→
11/03 01:00, , 4F
11/03 01:00, 4F
推
11/03 01:42, , 5F
11/03 01:42, 5F
→
11/03 01:43, , 6F
11/03 01:43, 6F
→
11/03 01:43, , 7F
11/03 01:43, 7F
→
11/03 01:44, , 8F
11/03 01:44, 8F
→
11/03 01:45, , 9F
11/03 01:45, 9F
→
11/03 01:45, , 10F
11/03 01:45, 10F
推
11/03 11:04, , 11F
11/03 11:04, 11F
→
11/03 11:04, , 12F
11/03 11:04, 12F
噓
11/03 11:38, , 13F
11/03 11:38, 13F
→
11/03 11:54, , 14F
11/03 11:54, 14F
→
11/03 11:55, , 15F
11/03 11:55, 15F
→
11/03 11:56, , 16F
11/03 11:56, 16F
→
11/03 11:56, , 17F
11/03 11:56, 17F
→
11/03 11:56, , 18F
11/03 11:56, 18F
→
11/03 12:01, , 19F
11/03 12:01, 19F
請問就是考慮重力對系統做功嗎?
但當符號定為△U = Q - W時,外界對系統作的功符號不是應該為負嗎?
另外egg break這件事情是否就不需要管他了?
推
11/03 12:08, , 20F
11/03 12:08, 20F
→
11/03 12:08, , 21F
11/03 12:08, 21F
→
11/03 12:28, , 22F
11/03 12:28, 22F
推
11/03 12:36, , 23F
11/03 12:36, 23F
→
11/03 12:37, , 24F
11/03 12:37, 24F
→
11/03 12:37, , 25F
11/03 12:37, 25F
→
11/03 12:38, , 26F
11/03 12:38, 26F
→
11/03 12:39, , 27F
11/03 12:39, 27F
→
11/03 12:40, , 28F
11/03 12:40, 28F
→
11/03 12:40, , 29F
11/03 12:40, 29F
※ 編輯: gnsh 來自: 111.184.227.135 (11/03 13:04)
推
11/03 13:01, , 30F
11/03 13:01, 30F
→
11/03 13:09, , 31F
11/03 13:09, 31F
推
11/03 17:45, , 32F
11/03 17:45, 32F
推
11/03 18:00, , 33F
11/03 18:00, 33F
→
11/03 18:00, , 34F
11/03 18:00, 34F
→
11/03 18:01, , 35F
11/03 18:01, 35F
→
11/03 18:01, , 36F
11/03 18:01, 36F
推
11/03 19:52, , 37F
11/03 19:52, 37F
→
11/03 19:53, , 38F
11/03 19:53, 38F
→
11/04 22:19, , 39F
11/04 22:19, 39F
→
11/05 00:55, , 40F
11/05 00:55, 40F
→
11/05 01:02, , 41F
11/05 01:02, 41F
因為我記得定義dU=Q-W,那外界作用給系統的功是負值,物體做的功是正值
不管是重力作功或蛋被外界打破,那功的符號不也應該是負值嗎?
板上高手怎麼判斷答案是(B)的呢? 我還是搞不懂QQ
※ 編輯: gnsh 來自: 111.184.227.135 (11/05 01:19)
推
11/07 18:41, , 42F
11/07 18:41, 42F
→
01/11 23:36, , 43F
01/11 23:36, 43F
→
01/11 23:39, , 44F
01/11 23:39, 44F
討論串 (同標題文章)