Physics / Unit 1: Motion in One Dimension
1.2 Speed and Velocity
Notes
Speed tells how fast; velocity tells how fast and in which direction.
- Average speed $\displaystyle = \frac{\text{total distance}}{\text{total time}}$ (scalar).
- Average velocity $\displaystyle v = \frac{\Delta x}{\Delta t}$ (vector).
- SI unit: meters per second ($\text{m/s}$). To convert km/h to m/s, divide by 3.6.
- Instantaneous speed is the speed at one moment, like a speedometer reading.
- Example: a cyclist rides 1,200 m in 240 s. Average speed $\displaystyle = \frac{1200}{240} = 5 \text{ m/s}$.
Practice
A student walks 150 m in 120 s. What is her average speed?
$\displaystyle \frac{150 \text{ m}}{120 \text{ s}} = 1.25 \text{ m/s}$.
A car travels at $72 \text{ km/h}$. What is this in m/s?
$72 \div 3.6 = 20 \text{ m/s}$ (1 km/h $\displaystyle = \frac{1000 \text{ m}}{3600 \text{ s}}$).
A swimmer swims 50 m down a pool and 50 m back in 80 s total. What are her average speed and average velocity?
Speed uses distance (100 m); velocity uses displacement (0 m).
Ready for more? Take the Unit 1 test.