Lesson focus: Engineering decisions need repeated evidenceObservations or measurements used to show whether an idea is right or wrong. and a stated success criterion.
Evaluate parachute designs using fair tests, anomalyA result that does not fit the pattern and should be checked. and constraintA limit or rule a design has to keep to, such as cost or size..
Build on earlier learning
Air resistance and egg dropUse drag and terminal-speed science to interpret design evidence.
You will be successful when you can…
- Identify the variables, including the control variables (the things you keep the same).
- Use repeated measurements.
- Identify an anomaly.
- Apply a success criterion and constraint.
Words that carry the science
- design criterion
- Measurable requirement defining success.
- constraint
- Limit a design must satisfy.
- repeatability
- Agreement when the same method is repeated.
- anomaly
- Result inconsistent with the wider pattern.
- trade-off
- Improvement in one outcome accompanied by a cost elsewhere.
Improve a design from evidence, not preference
01State success first
“Best” has no meaning without criteria such as longest repeatable fall time while staying below a mass limit.
02Test fairly
Change canopy area while holding mass, drop height, line length and release method constant. Repeat because air movement creates variation.
03Use anomalies cautiously
Investigate an unusual result; do not delete it automatically. Report why it may be unreliable and repeat.
An anomalyA result that does not fit the pattern and should be checked. (odd result) is a result that does not fit the others. If two times are close together and one is very different, the odd one may be a mistake, such as letting go badly or timing late. Repeat the test before you decide which design is best.
Falling with air resistance
Air resistance grows as the skydiver speeds up. When it balances the weight, the speed stops changing: terminal velocity. Opening the parachute makes the air resistance much bigger.
Controls
Choose within a constraint
Design C has longest times but mass 14 g; limit is 12 g. Design B has two times near 2.5 s and one 1.2 s.
- Reject C for violating mass constraintA limit or rule a design has to keep to, such as cost or size..
- Identify B trial 3 as anomalous.
- Use repeatable B values, about 2.55 s.
- Recommend B with another repeat and a secure release check.
How to make the reasoning visible: Constraints can outweigh a single performance measure.
Design evaluation
Use the figure data and these control variables.
| Issue | Evidence | Action | Reason |
|---|---|---|---|
| B anomaly | 1.2 s vs 2.5/2.6 s | Repeat B | Check release failure |
| C mass | 14 g | Reject/redesign | Exceeds 12 g |
| Canopy area | 20, 30, 40 cm | Keep shape/material | Isolate area |
| Wind | Variable | Test indoors first | Improve control |
Why not average B immediately?
Which design meets the stated criterion best?
Use the table. Why is it better to test the egg-drop designs indoors first?
Check your understanding
Answer from memory, then check against the suggested answer.
What is design criterion?
What is constraint?
What is repeatability?
What is a trade-off in design?
Why should a test be repeated?
Suggest one control variable in a canopy-area test.
Why may an indoor winner fail outdoors?
To test the effect of canopy area, which control variables should stay the same?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Three drop tests of paper-helicopter design B gave fall times of 2.5 s, 2.6 s and 1.2 s. Answer the parts to evaluate the results.
Which two times are close together?
(1)Which time is the anomaly (odd result)?
(1)Suggest one mistake that could have caused the odd result.
(1)What should the student do next?
(1)Finish the sentence: “Design B looks … but …”
(1)Total for this question: 5 marks
Command word: For evaluateA command word: weigh up the good points and bad points, then give your overall judgement., look at the evidence, spot problems, then give a careful judgement.