When a particle moves along a straight line, we describe its position with respect to an origin O by means of a coordinate such as x. The particle’s average x-velocity v_av-x during a time interval Δt = t₂-t₁ equal to its displacement divided by Δt The instantaneous x-velocity at any time t is equal to the average x-velocity for the time interval from t to t+Δt in the limit that Δt goes to zero. Equivalently, is the derivative of the position function with respect to time. (See Example 2.1.)

\frac{1.5 \times 10^{6} V/m}{3 \times 10^{8} V/m}

\frac{x_{2}}{y}

\frac{x_{2} - x_{1}}{y}

v_{av-x} = \frac{x_{2} - x_{1}}{t_{2} - t_{1}}

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v_{av-x} = \frac{x_{2} - x_{1}}{t_{2} - t_{1}} = \frac{Δx}{Δt}

v_{x} = lim Δt->0 \frac{Δx}{Δt} = \frac{dx}{dt}