Study Guides/Physics/Average Velocity Formula and Definition
Study Guide · Physics

How to Calculate Average Velocity

In kinematics, velocity is not the same as speed. While speed tells you how fast an object is moving, velocity tells you how fast it is moving and in what direction. Calculating the average velocity over a journey is a fundamental physics concept.

Question (Click to Flip)

If I drive 50 km East and 50 km West back home, what is my average velocity?

Answer

Since you returned to your starting point, your total displacement is zero. Therefore, your average velocity for the entire trip is 0 km/hr.

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Key Facts

Velocity is a vector quantity; it has both magnitude and direction.

Average velocity can be zero or even negative, but average speed is always positive (or zero if at rest).

A negative average velocity simply means the net movement was in the opposite direction of the chosen axis.

1. The Standard Formula

Average Velocity is defined as the total displacement divided by the total time taken.

Average Velocity (v) = $\frac{\text{Total Displacement (\Delta x)}}{\text{Total Time Taken (\Delta t)}}$

  • Displacement is the shortest straight-line distance from the starting point to the ending point (Final Position - Initial Position).
  • The SI unit for average velocity is meters per second (m/s).

2. Formula for Constant Acceleration

If (and ONLY if) an object is moving in a straight line with a constant uniform acceleration, the average velocity can be calculated by simply averaging the initial and final velocities:

Average Velocity = $\frac{\text{Initial Velocity (u)} + \text{Final Velocity (v)}}{2}$

3. Average Velocity vs Average Speed

Imagine you run one lap around a 400m circular track in 100 seconds.

  • Average Speed = Total Distance / Time = 400m / 100s = 4 m/s.
  • Average Velocity = Total Displacement / Time. Since you ended up exactly where you started, your displacement is 0. Therefore, Average Velocity = 0 m/s.

Questions and Answers

If I drive 50 km East and 50 km West back home, what is my average velocity?+

Since you returned to your starting point, your total displacement is zero. Therefore, your average velocity for the entire trip is 0 km/hr.

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