※ 引述《Ertkkpoo (water)》之铭言： : 想请问一个很基本的问题 : F ______________ : → | | : | | f : ___________________ ← : \\\\\\\\\\\\\\\\\\ : 目前物体不动，故f是静摩擦力 : 如果就水平方向来写equation of motion : 根据 F=ma : x : 那我应该写成 F-f=m*a_x : 还是写成 F-f=0 呢? : 如果写成F-f=0不就代表我已经把答案解出来了? : 其实我主要是想问到底equation of motion要写到怎样的程度呢? : .. : 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，这样就好吗? : 谢谢大家 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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