※ 引述《Ertkkpoo (water)》之銘言： : 想請問一個很基本的問題 : F ______________ : → | | : | | f : ___________________ ← : \\\\\\\\\\\\\\\\\\ : 目前物體不動，故f是靜摩擦力 : 如果就水平方向來寫equation of motion : 根據 F=ma : x : 那我應該寫成 F-f=m*a_x : 還是寫成 F-f=0 呢? : 如果寫成F-f=0不就代表我已經把答案解出來了? : 其實我主要是想問到底equation of motion要寫到怎樣的程度呢? : .. : 只要列出所有X方向的力，做一下向量和，然後a_x也可以改成x 或不改，這樣就好嗎? : 謝謝大家 Just the way im thinking, when i think of EoM, it seems to be x"(t)=something. (a second order differential equation) Therefore, in your case, mx"=F(net)=ma →mx"=ma=0 →x"(t)=0 (this is the equation of motion) and we can solve for the trajectory from here, namely x'(t)=C (C is a constant depends on initial condition. To be more specific, in your case, the obeject is stationary, therefore, C=0) x(t)=Ct+B (B and C are both constants, and both of them depends on initial condition. Follow by what we said in previous calculation for x'(t)=C=0, we have x(t)=B. This make sense since as time pass by, the object remains at the same spot and it doesnt move) Hope this explanation clears off some confusion, if not, we can discuss more --

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