大家有無咩關於physics嘅問題但又無地方問

咁如果我係中間掘出一房咁大嘅洞而我又成功走到入去 咁我到時會係痴住個洞嘅邊緣黎走定係維持係中間

咁如果我喺架地鐵上面行嘅話咁我咪行得快過地鐵

問題一，點解宇宙會不停擴張，抱歉我唔知咩係宇宙大爆炸，如果係爆炸，個force都應該有會停的一日

問題2，如果不斷擴張，睇你之前講係地破
球同其他星球距離都遠左，咁簡單黎講，佢係內部擴張而唔係外圍空間越來越多，咁即係最外圍新擴張的地方其實係無野，唔會多左星球？只係原有數量的星球而距離遠左？

唔太明，我知宇宙一直膨脹緊，但無邊界又點會有「膨脹」呢個概念？定還是「膨脹」唔係指邊界嘅擴張？

即係總之啲 static star 離我地越嚟越遠（red shift）就叫做「膨脹」，同有無邊界係無關係？

利申：AL phy+ UG係咁二讀過下 intro. to astronomy

因為G field係零，所以你個人本身喺邊就會喺邊
你喺門口入去就喺門口，瞓喺中間就喺中間，除非有外力影響
你當自己係太空人就得

A riderless bicycle can automatically steer itself so as to recover from falls. The common view is that this self-steering is caused by gyroscopic precession of the front wheel, or by the wheel contact trailing like a caster behind the steer axis. We show that neither effect is necessary for self-stability. Using linearized stability calculations as a guide, we built a bicycle with extra counter-rotating wheels (canceling the wheel spin angular momentum) and with its front-wheel ground-contact forward of the steer axis (making the trailing distance negative). When laterally disturbed from rolling straight, this bicycle automatically recovers to upright travel. Our results show that various design variables, like the front mass location and the steer axis tilt, contribute to stability in complex interacting ways.

佢話common view係
gyro+caster令單車可以self steer

而佢就整咗個新設計
都做到self steer
所以結論係
“neither gyro or caster is necessary for…”

所以佢其實冇否定普通單車
gyro+caster呢個解釋？
我有冇get錯

澎漲緊
可唔可用一個假設係 在一個空間內expand緊？

PDF

http://scienceblogs.de/geograffitico/wp-content/blogs.dir/70/files/2012/07/i-df8fe9b1809387dc1eab0b693d2bcf70-StableBicyclev34Revised.pdf

理論上係
但係拎住電筒跑啲光唔會快過光速，因為光速牽涉到相對論，個計法唔係就咁加埋。
https://en.wikipedia.org/wiki/Velocity-addition_formula

明白曬，唔該師兄

１）點解宇宙會不停膨脹...呢個仲係不解之謎
以現時主流說法（ΛCDM Model）嚟講，係有一種不知名嘅能量令佢膨脹，呢種能量就係成日講嘅Dark energy。無人知佢係咩，甚至可能有一日發現ΛCDM Mode係錯，根本無Dark energy都唔出奇。
至於宇宙最後係點都係未知，wiki下面舉咗幾個例，當故睇都幾有趣
https://zh.wikipedia.org/wiki/%E5%AE%87%E5%AE%99%E7%9A%84%E7%B5%82%E6%A5%B5%E5%91%BD%E9%81%8B

２）無邊際所以無外圍呢個概念，只係地球同鄰居會愈來愈遠，最後遠到見唔到鄰居

係表面
其實用氣球都多bug，用冬甩會仲好

General Relativity已經超過咗UＧ程度，我就唔衝出嚟柒啦
你睇下呢兩條片幫唔幫到你
https://www.youtube.com/watch?v=eNhJY-R3Gwg
https://www.youtube.com/watch?v=tzQC3uYL67U

我都唔知
除咗用嚟計數仲有咩用

睇完
以我理解，佢個reasoning係
要講self stability呢
就要睇晒所有lean-steer coupler

於是佢搵晒有lean steer關係嘅factor
然後設計上將gyro同caster變做0

發現冇咗gyro同caster都stable
即係有其他factor都關事
所以結論係stability唔係單靠呢兩樣野
更加唔係必需要呢兩樣野
而係所有lean-steer factor交雜嘅成果

留意返 ”set zero仍然stable”
只能推論 ”not necessary”
而唔係 ”irrelevant”

而佢都確實冇否定過啲咩
佢最後嗰段都係bbq happy ending添

Although we showed that neither front-wheel spin angular momentum nor trail are necessary for self-stability, we do not deny that both are often im- portant contributors. But other parameters are also important, especially the front-assembly mass dis- tribution, and all the parameters interact in complex ways.

力矩既「方向」其實只係個平面。講緊旋轉既平面：例如喺xyz空間，z方向既力矩其實只係指個xy-plane。平面係2D，但咁啱空間係3D，3-2=1，一個平面既量可以用1D 向量（方向）表示。即個力矩喺xy plane，我地就用力矩不存在既維度：z，表示。三維入面，一個2D平面永遠可以用1D向量代表。但個根本物理其實係個2D平面（即例子中既xy plane)。

如果個空間並非向三維，咁力矩及其他旋轉有關既物理就唔可以用方向表示。

實際上個數可以唔用方向，直接用個二維平面呢個數學物件表示，叫2-form。所有旋轉既物理既公式，都可以用2-form表示，而唔洗用方向。平時力矩或者角動量個方向，係旋轉平面個2-form既Hodge dual。一個N維平面喺D維空間既Hodge dual係（D-N）維平面。如果D=3, N=2，咁啱就係一支向量。

Quantum mechanics 入面既particles係representations of (c) spin groups，govern物理系統演化既schoedinger equation (或者path integral) 可以用一個functional叫Action表示，有時候個Action要invariant under transformation of spin groups (elements of the unitary representations)

好似係