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Why do this problem?
This problem brings together several mathematical ideas: modelling, using quadratic graphs to solve problems, rearranging quadratic formulas, and interpreting results in the context of a real-world problem.
Possible approach
This problem follows on from
Reaction Timer. Perhaps spend some time working on the second experiment before working on this problem.
Begin the lesson with this question on the board:
If two people caught the ruler at 15 and 30cm do you think the first person's reactions are twice as fast as the second person's?
Give learners a chance first to think about it on their own, and then to agree an answer with their partner, before discussing it with the rest of the class.
Then pose the question:
What is the relationship between the distance travelled by the ruler and the reaction time?
Allow a short time for learners to speculate about what sort of relationship they might expect to discover. Then hand out this worksheet (
Word,
pdf) for learners to work on the problem in pairs.
If necessary, bring the class together to talk about units of acceleration and velocity and explain the notation $ms^{-1}$ and $ms^{-2}$, and to clarify any other new ideas met in this task.
Towards the end, give the class time to carry out the experiment and see how their reaction times compare with the average.
Key questions
As an object falls, when is it travelling slowest/fastest?
What does the graph of distance against time tell you about the change in distance when the time doubles?
Do you think the modelling assumptions are reasonable?
Possible extension
The Stage 5 problem
Cannon Balls explores vertical motion of a cannon ball and solving equations of vertical motion.
When considering the validity of the modelling assumptions, learners could sketch how they think the graph might be changed if air resistance was taken into account.
Possible support
Work together as a class filling in the first few values of the time/distance table.