Motion and forces
Distance, displacement, speed and velocity
Separate distance from displacement and speed from velocity, work out average speed, average velocity and average acceleration, and say what stays fixed in uniform circular motion.
An athlete runs along a straight track. To say where she is, you first fix a point to measure from — her starting line — and then give her distance and direction from it. That fixed point is the reference point, and she counts as moving when her position with respect to it changes with time.
Two pairs, not four ideas
Distance travelled is the whole length of path she covers; displacement is the net change from where she began, and it carries a direction. Divide each by the time interval and you get the other pair: average speed from distance, average velocity from displacement. So the direction that displacement has, average velocity has too — and distance and average speed have none.
Have a play
Slide along the track. The starting line sits at 0 and the far end at 100.
Tap anywhere on the line.
Worked example
The athlete runs from the start out to the 100 m mark, then turns and runs back to the 60 m mark. How far has she travelled, and what is her displacement?
Add the two legs of the path: 100 m out, then 40 m back, so the total distance travelled is 140 m.
Distance travelled counts the whole path she covered, including the turn. It does not care that some of it retraced her steps.
Try it together
Now work out an acceleration from a bus's two velocities.
A bus is doing 36 km/h. The driver presses the accelerator for 10 s and it reaches 54 km/h. Answer with a bare number each time.
1.First put the starting velocity into SI units. What is 36 km/h in m/s?
Have a go
Have a go on your own. A car slows from 20 m/s to rest in 4 s. What is the magnitude of its average acceleration, in m/s²?
Ready to practise?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.