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Sports Physics 6-8 / Week 12 / Tuesday
2/6
Week 12 · Design a Landing Pad

Tuesday

Plan a fair test
// Criteria, constraints, build, test, improve
⏱ about 20 min

Tuesday: Plan a Fair Test

Rocket has a cardboard tray on the patio already half full of folded paper. "Let me just drop it and see!"

Raven holds up the Field Log. "Drop it from where? Onto what? How many times? If every drop is different, what did we learn?"

Rocket lowers the ball. "Same height every time. Same ball. Three drops. Then change one thing."

"What do you notice about that plan?" Raven asks. "It is the same shape as every Field Lab this term."

Nova hovers over the tray. "Engineers test, then change, then test again," she says. "Each round is an iteration."

Rocket grins and finds a tape mark at his shoulder height. "Then this is drop height. Written down. Not changing."

The design cycle

The engineering standards say a solution needs to be tested, then modified on the basis of the test results, to improve it.

They also describe an iterative process. Testing the most promising solutions and modifying them based on results leads to greater refinement.

That cycle has a shape. Define the problem, build a prototype, test it fairly, change one thing, test again.

Define criteria and constraints
Build the first prototype
Test it with a fair test and record the data
Compare the data with the criteria
Change one thing
Test again and compare

What makes a test fair

A fair test changes only one thing at a time and repeats each trial. You used this rule in every Field Lab, including last week's angle test.

This week the fixed things are the drop height, the ball, the floor spot and the ramp. The thing you change is the pad.

Your first test is a baseline: drop the ball onto the bare floor and roll the car onto the bare floor. Then test the pad the same way.

A physics text compares bounce height with drop height for a ball dropped on a hard floor. That ratio measures how lively the bounce is.

Its numbers work out to about 72 percent of the drop height for a new tennis ball on a hard court. Your pad should do far less.

ThingFixed or changed?How you keep it fixed
Drop heightfixeda tape mark at shoulder height, never higher than you can reach standing
The ball and the toy carfixedthe same ones every trial
Ramp height and positionfixedone book, taped in place
Number of trialsfixedthree drops and three rolls per test
The padchangedbare floor first, then your prototype, then one change
FAIR OR NOT FAIR?
  • Read the question.
  • Tap your answer.
You drop from shoulder height onto the floor, then from head height onto the pad. Fair?
Why drop three times instead of once?
Which test comes first?
THE DESIGN CYCLE, IN ORDER
  • ?Run a fair test and record the data.
  • ?Define criteria and constraints.
  • ?Build the first prototype.
  • ?Compare the data with the criteria.
  • ?Change one thing and test again.
WHY THIS EXERCISEEngineers follow this order so that every change is judged against the same criteria.
A fair test changes only ____ thing at a time.
The first test, on the bare floor, is called the ____.
StatementTrue or false?
A solution should be tested and then modified on the basis of the results.?
Changing the pad and the drop height together is a fair test.?
Each round of test, change and test again is an iteration.?
A new tennis ball on a hard court bounces back higher than it was dropped.?
WHY THIS EXERCISETomorrow's build only counts as engineering if the test around it is fair.
Try it
Pick your drop spot and put a tape mark at shoulder height on a wall or post beside it.
Set the ramp (one book) on the floor and tape its edges down. Measure 1 metre of floor beyond the ramp end and mark it.
Write both setups in your Field Log with the numbers. They do not change all week.
Draw your test setup from the side: the tape mark, the drop spot, the ramp and the measuring line.

Solid plan. Tomorrow is Field Lab: build the prototype and run the tests.

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