physics work問題

The given answer (option A) is incorrect. A possible reason is that the incorrect given answer is based on thecalculation of an initial speed of 0 m s⁻¹ and a final speed of 15 ms⁻¹.

Overall speaking, your calculation is alright, but modification is needed :
2 a (300) = 25² - 15²
a = 2/3 ms^-2
1200 g sinθ - f = 1200 (2/3)
1200 (10) (1/10) - f = 1200 (2/3)
f = 400 N ..... (no correct answer)

The steps given by Wong are correct, but the answeris incorrect :
PE(decrease)=KE(increase)+ work done by friction
m g h = 0.5 m (v² - u²) + Fs
1200 x 9.8 x 30 = 0.5 x 1200 x(25² - 15²) + F(300)
F = 376 N
Taking g as 10 m s⁻¹, F = 400 N
(The same answer is obtained.)

2012-07-21 22:28:52 補充：
I know the reason now. The first paragraph in my answer should be changed to :

"The given answer (option A) is incorrect. The reason is that the incorrect given answer is based on thecalculation of an initial speed of 20 m s⁻¹."

2012-07-21 22:31:58 補充：
Both methods of using "conservation of energy" and "motion of uniform acceleration" are suitable to solve this type of question. "Conservation of energy" is easier to understand than "motion of uniform acceleration", but NOT more suitable.

2012-07-21 22:33:56 補充：
Wong and I gave different answers. Wong's answer is 750 N, but my answer is 400 N.

2012-07-21 22:35:50 補充：
Besides, I wonder why Wong got the wrong answer of option A by using a correct calculation method.

2012-07-21 22:38:08 補充：
The give solution is NOT totally wrong, but just partially wrong. The only mistake is using 20 m s⁻¹ as the initial speed.

2012-07-23 20:57:24 補充：
你的方法沒有錯，只是 Wong 說你錯而已。

答案應是 400 N，與你的答案 393 N 接近。差異在於你把 1/10 轉換成 sin 5.71°（應是 5.73°），其實無需轉換，直接用 sinθ = 1/10 便可。

至於所提供的答案（A）錯誤，是因為初速用錯了 20 ms⁻¹（正確是 15 ms⁻¹）。

由於是問 average friction (平均摩擦力)，故把 friction 當成 constant。因此，可作 motion of uniform acceleration (勻加速度運動) 計算。

首先... 你第一步起手就錯左拉已經.... 因為v^2-u^2=2as 呢題formula系for uniform acceleration motion...所以全部計算都錯曬... concept未夠清...
通常講親average friction 通常都會由work done 去入條題。
我斗左條題好耐，比你玩死，原來個g=9.8，not10，你唔講啊...

好容易用三角比例可以計算到vertical displacement =30m
PE(decrease)=KE(increase)+ work done by friction
mgh = 0.5m(v^2-u^2) + Fs
1200x9.8x30=0.5x1200x(25^2-15^2)+F(300)
F=752N
甘約等於750N

希望可以幫到你... concept方面要加強

2012-07-21 14:54:12 補充：
sorry! your solution is total wrong! how to find the velocity of 20ms^-1?? it should be 15ms^-1. Hence,

賣女孩的火柴 and my answer is correct.

2012-07-21 14:56:50 補充：
although the answers are the same , I think using the concept of energy to solving this question is more suitable than using concept of motion!