;; This buffer is for text that is not saved, and for Lisp evaluation. ;; To create a file, visit it with C-x C-f and enter text in its buffer. how much thing count value ----- ----- 3 80 2 90 average = (80 + 80 + 80 + 90 + 90) / 5 = ( 3 * 80 + 2 * 90 ) / ( 3 + 2 ) = sum ( weight_i * value_i ) / sum ( weight_i ) mass x ---- -- 3 80 2 90 M = m1 + m2 = total mass X = center of mass position x1 , x2 individual positions m1, m2 individual masses center of mass = average position = X = (m1 * x1 + m2 * x2) / M M * X = m1 * x1 + m2 x2 d (M X) d( m1 x1) d (m2 x2) ------- = --------- + -------- dt dt dt M V = m1 v1 + m2 v2 P = total momentum = M * V_cm = sum(p_i) = (p_1 + p_2) V = velocity of center of mass (X,Y) = ( m1 (x1,y1) + m2 (x2,y2) ) / M - - - - - - - - - - - - - - - - - - - - - -