FHSST Physics/Rectilinear Motion/Speed and Velocity
Speed and Velocity
Let's take a moment to review our definitions of velocity and speed by looking at the worked example below:
Worked Example 23 Speed and Velocity
Question: A cyclist moves from A through B to C in 10 seconds. Calculate both his speed and his velocity.
Step 1 :
Analyse the question to determine what is given. The question explicitly gives
- the distance between A and B
- the distance between B and C
- the total time for the cyclist to go from A through B to C
all in the correct units!
Step 2 :
What is being asked? We are asked to calculate the average speed and the average velocity of the cyclist.
His speed - a scalar - will be
Since velocity is a vector we will first need to find the resultant displacement of the cyclist. His velocity will be
The total displacement is the vector from A to C, and this is just the resultant of the two displacement vectors, i.e.
Using the rule of Pythagoras:
For this cyclist, his velocity is not the same as his speed because there has been a change in the direction of his motion. If the cyclist traveled directly from A to C without passing through B his speed would be
and his velocity would be
In this case where the cyclist is not undergoing any change of direction (i.e. he is traveling in a straight line) the magnitudes of the speed and the velocity are the same. This is the defining principle of rectilinear motion.
|For motion along a straight line the magnitudes of speed and velocity are the same, and the magnitudes of the distance and displacement are the same.|