人在行駛中的火車上跳起會退後嗎?(和引申出來的問題)

jackiema

Cba to type in Chinese... Because there are loads of cases of take care of.

原帖由 江戶川柯南回歸 於 2006-12-12 20:01 發表
問題1:人在行駛中的火車上跳起會退後嗎?
問題2:假設一樣物件停留在火車中，而沒有接觸任何火車上的部件，那火車行駛的時候這物件會退後嗎?
問題3:一隻鳥在火車上以 y 的速率飛行，而火車的速率是x~那鳥的速率會變成是x+y嗎?

1) If the train travels at constant speed, and the man jumps, the man will land at where he jamp. If the velocity of the train is v, the man travels at the velocity of v as well. Therefore, when he jumps, he will travel together with the train at v = v
    If the train accelerates when the man jumps, the poor man will hit the back of the train. Let the velocity of the train when the man jumps be u. The man jump and travels at the velocity of u. The train then accelerates to verlocity of v. Relating to the train, the man's velocity is u - v < 0. Therefore, the man hits the wall at the back of the train.
    One fun thing, if the train starts turning at the moment right when the man jumps, the man will hit the side wall at the back of the train. Think about why.
2) If you mean that the object doesn't even touch the train (i.e. no gravity acting on the train), then the object will get to the back of the train.
    If you mean that the object touches train, but not anything else, the object will either get to the back of the train, or the friction prevent the train from moving.
3) If the bird take off, and travels at v = y while the train accelerates to v = x, then the bird travels at the velocity of x to the surrounding, and velocity of x - y to the train.
    If the bird take off when the train is already at v = x, and the bird accelerates to v = y, the bird travels at the velocity of x + y to the surrounding, and velocity of y to the train.
P.s. I am just a form 4 student and just studying velocity / acceleration / displacement / distance, so I am not sure if I got the explaination correct. (However, I have self-studied till inertia now --)
P.s.2 If you are studying in form 4+, then you should study about inertia more...

原帖由 江戶川柯南回歸 於 2006-12-12 20:36 發表

無可能吧~
如果時間係短既話仲有可能~
但係時間一長這力沒有可能一直保留吧~

It will.
The air friction acting on the man when he jumps in the train is very small (as the air molecules travel at the same speed as the train do).

[ 本帖最後由 jackiema 於 2006-12-14 19:07 編輯 ]

jackiema

原帖由 [S]鷹眼米霍斯 於 2006-12-14 19:14 發表
任何正常既物體都唔會冇啦啦有速度，一定要比力先有
所以一樣事物唔會無啦啦向後
只不過受到火車向前行既影響，你會覺得物件向後
就好似d樹緊，你o係火車到睇好似佢地退緊後咁，事實好明顯唔係
understand?

錯。
當我坐火車, 路旁的樹相對於我來說是在動的。

原帖由 痔痕仔@恆 於 2006-12-14 21:10 發表
第3題唔係x+y
如果係咁既話, 一支每秒鐘可以行10萬公里既火箭, 係一支每秒行十萬公里既火箭上面發射, 咁果支火箭咪變左每秒鐘20萬公里? 再整多支咪變光速??

所以愛因斯坦為左針對依個問題, set左以下既公式 ...

但相對於周圍, 鳥兒的速度的確是 x + y
以我所知, 速度愈接近光速, 質量愈大。
到達光速時, 質量無限。

[ 本帖最後由 jackiema 於 2006-12-14 22:01 編輯 ]