Velocity vs. Speed: Understanding the Difference
In everyday conversation, the words "speed" and "velocity" are often used interchangeably. However, in physics, they describe two very different concepts.
- Speed is a scalar quantity. It simply tells you how fast an object is moving. If you are driving a car at $60 , \text{mph}$, your speed is $60 , \text{mph}$. It does not care where you are going.
- Velocity is a vector quantity. It tells you both how fast an object is moving and the specific direction it is moving in. Velocity requires displacement, not just raw distance.
If you drive $60 , \text{mph}$ directly North, your velocity is "$60 , \text{mph} , \text{North}$." If you drive your car in a perfect circle and end up exactly where you started, your average speed might have been $60 , \text{mph}$, but your average velocity is zero, because your total net displacement is zero.
Calculating Constant Velocity
This calculator determines average constant velocity based on the total displacement of an object over a specific period of time.
The Formula
The fundamental mathematical definition of velocity ($v$) is the change in position (displacement, $\Delta x$) divided by the change in time ($\Delta t$).
Example Calculation
Imagine a sprinter runs down a perfectly straight track, covering a displacement of 100 meters in exactly 10 seconds.
- Velocity: $v = \frac{100}{10} = \mathbf{10 , \text{m/s}}$
- Conversion to km/h: To convert meters per second to kilometers per hour, you multiply by 3.6. Therefore, $10 \cdot 3.6 = \mathbf{36 , \text{km/h}}$.
Positive and Negative Velocity
Because velocity is a vector, it can be negative. If we define "forward" (or moving to the right on a graph) as the positive direction, then an object moving forward has a positive velocity. If an object is moving backward (or to the left), it has a negative velocity.
For example, if you back your car out of a driveway at $5 , \text{m/s}$, your speed is $5 , \text{m/s}$, but your velocity is $-5 , \text{m/s}$ relative to your forward-facing starting position.