Idea 5of 6Builds onTwo readings of any graph
Two pings
How does a radar gun know your speed?
Today you'll discover
- the one measurement a speed needs
- why two positions and a time are enough
- what the number cannot tell you
Think firstA radar gun pings a cyclist twice
A radar gun sends two pulses a moment apart. What does it actually measure?
Decide first, then press Next. Two points, then the slope between them.
An empty position–time graph. After a prediction, two points appear joined by a line, twelve metres apart and three seconds apart. Then the slope triangle reads four metres each second. The last step asks what the gun cannot tell from two pings.
See the whole ideathe big idea and the key word, in one place
The big idea
Two positions and a time: a speed. Nothing in between.
Where it goes nextA measurement between two moments is an average over that gap. Instruments that ping faster see finer detail, and never the instant itself.
About this modelThe line is not the path. Up means further east, not uphill; a falling line means walking back, not walking downhill.
The line is not the path. Up means further east, not uphill; a falling line means walking back, not walking downhill.
Ready to use it?
- Explain what a two-ping speed measurement does and does not know.