Speed tells you how far something travels in a certain time. Speed = distance ÷ time. If distance is in metres and time in seconds, speed is in metres per second (m/s). The same formula rearranges: distance = speed × time, and time = distance ÷ speed. Most journeys change speed, so we often work out the average speed for the whole trip.
A distance–time graph shows a journey. A straight sloping line means a steady speed, and a steeper line means a faster speed. A flat, horizontal line means the object is stationary: time passes but the distance does not change. The gradient (slope) of the line equals the speed.
Worked example
A cyclist travels 300 m in 60 s. What is her average speed?
- Speed = distance ÷ time.
- 300 ÷ 60 = 5.
Answer:Her speed is 5 m/s.
Key words
- speed
- distance travelled per unit of time
- average speed
- total distance ÷ total time
- stationary
- not moving
- gradient
- the steepness of a line
New questions every time.
NGSS-referenced: MS-PS3-1