HS-ETS1-2: Breaking a Complex Problem into Solvable Engineering Problems
In plain English: HS-ETS1-2 is the NGSS high school performance expectation that asks students to design a solution to a complex real-world problem by breaking it into smaller problems that engineering can solve. Students give each piece its own criteria and constraints, solve the pieces, then check that the parts work together. It is usually taught inside a high school science or engineering course.
Design a solution to a complex real-world problem by breaking it down into smaller, more manageable problems that can be solved through engineering.
Next Generation Science Standards · Disciplinary Core Idea: Engineering Design · Official standard
Students learn to take a problem that is too big to solve in one step, such as a flooding parking lot or an overheated portable classroom, and break it into smaller problems that engineering can solve one at a time. For each subproblem they write its own criteria and constraints, split the shared budget (money, mass, energy or space) among the pieces, solve the pieces with simple calculations, and then recombine them to check that the whole design still meets the original criteria. The practice is Constructing Explanations and Designing Solutions, and the core idea is ETS1.C, Optimizing the Design Solution: large criteria can be split into simpler ones that are handled systematically, and trade-offs between criteria may be needed.
NGSS gives no Clarification Statement and no Assessment Boundary for HS-ETS1-2, so this lesson stays with the wording of the performance expectation: every task asks students to decompose a real problem, solve engineering subproblems and recombine them. All site data, prices and test results on this page are invented for teaching and labeled as invented.
Learning Objectives
By the end of this lesson, students will be able to:
Break a complex real-world problem into smaller subproblems that can be solved through engineering, and separate them from parts that need rules or behavior change
Write a measurable criterion and a constraint for each subproblem that follow from the criteria and constraints of the whole problem
Allocate a shared budget of cost, mass, energy or space among subproblems and identify the interfaces where sub-solutions must match
Solve individual subproblems with simple calculations and use data to recombine the sub-solutions and test them against the whole-system criteria
Revise a decomposition when a subproblem cannot be solved inside its constraints
Prior Knowledge Required
Students should already be comfortable with:
Defining criteria and constraints for a design problem MS-ETS1-1
Comparing design solutions against criteria and constraints MS-ETS1-2
Working with units of area, volume, power and energy, including watts and watt-hours
Percentages and rates, such as liters per minute and people per hour
Put one big goal on the board and give students three minutes alone, then three minutes with a partner.
Warm-Up Prompt
"The school wants to cut cafeteria food waste in half by June. Write down every separate thing that would have to change for that to happen. Which of those things could an engineer design, and which could not?"
Collect answers on sticky notes and group them on the board: measuring the waste, portion sizes, storage and cooling, composting, the lunch schedule. Point out that nobody wrote one single solution: the goal only became workable once it was split up. Also point out the notes that are not engineering problems (a new rule about taking extra food, a lunch schedule change). HS-ETS1-2 asks students to find the pieces that engineering can solve.
Direct Instruction20 minutes
What makes a problem complex. A complex problem has several parts that interact, several criteria at once, and shared limits on money, space, time or energy. Engineers handle it by decomposition: splitting it into subproblems that are small enough to design, build and test on their own, while keeping track of how the pieces connect. Use Diagram 1 as the model for the steps below.
State the whole problem and its system-level criteria and constraints: what must be true when the whole design works, and what limits apply to everything.
Split by function: ask what the system must do (collect, store, move, protect, measure). Each function becomes a candidate subproblem.
Give every subproblem its own measurable criterion and constraint, derived from the whole-system ones. If you cannot write a number or a test for a piece, split it again or rewrite it.
Separate what engineering can solve from what needs a rule, a schedule or a change in behavior. Keep those on the list, but do not design devices for them.
Allocate the shared budget: the money, mass, energy or area given to the subproblems must add up to no more than the total.
Name the interfaces: the output of one piece is the input of another (a volume, a flow rate, a power draw, a size). Teams must agree on these numbers before they design.
Solve, recombine and test the whole: add the sub-solutions back together and check them against the system-level criteria. If the whole fails, find the subproblem or interface that caused it and iterate.
Worked example: a flooding parking lot (invented site data). After heavy rain, water stands on the school's 2,000 m² back parking lot and the path to the gym. The design storm is 25 mm of rain, asphalt sends about 90% of the rain that falls on it into runoff (a runoff coefficient of 0.9), the lawn strip beside the lot has 200 m² available, ponded water may be at most 0.15 m deep for student safety, and the budget is $12,000. Useful fact: 1 mm of rain on 1 m² is 1 liter, so 25 mm on 1 m² is 0.025 m³.
Subproblems of the parking lot design
Subproblem
Criterion
Constraint
1. Estimate volume
Runoff volume for the 25 mm storm is known in m³
Uses the site area and runoff coefficient
2. Store it
Stores the whole runoff volume
At most 200 m² of lawn; ponding at most 0.15 m
3. Move the water
All flow reaches storage
No erosion at curb cuts
4. Keep it safe
Ponded water drains within 24 hours
Maintenance twice a year
Decompose the problem
Split "the back lot floods" into subproblems by function, each with a criterion and a constraint.
Result: Four subproblems: estimate the volume, store it, move the water to storage, keep the stored water safe (see the table)
Solve subproblem 1: the volume
Runoff volume = area × rain depth × runoff coefficient = 2,000 m² × 0.025 m × 0.9.
Result: V = 45 m³ of runoff to handle in the design storm
Solve subproblem 2 inside its constraint
A rain garden 0.15 m deep would need 45 ÷ 0.15 m² of area, but only 200 m² is available.
Result: Needs 300 m²; 200 m² × 0.15 m holds only 30 m³, so 15 m³ is still unstored
Revise the split and recombine
Add a new subproblem: a 15 m³ underground storage chamber. Garden at $35 per m² for 200 m², chamber at $300 per m³.
Result: 30 + 15 = 45 m³ stored; $7,000 + $4,500 = $11,500, within the $12,000 budget
Stress the move in the third example: the storage subproblem could not be solved inside its own constraint, so the class did not break the safety limit by digging deeper. It changed the decomposition, and then checked that the recombined design still met both whole-system criteria. Students often skip that check.
Guided Practice15-20 minutes
Groups of three decompose a second problem with you: a portable classroom reaches 33 °C on hot afternoons and the district cannot add air conditioning this year. Show Diagram 2, the invented heat gains at 3 pm. Each person gives off about 100 W of heat while seated, so 25 students and a teacher add 2,600 W.
Heat gains in the portable classroom (invented data)
Source
Heat gain (W)
Proposed sub-solution
Expected cut
Roof (sun)
4,000
Reflective roof coating
50% of roof gain
Windows (sun)
2,000
Exterior shades on the west windows
60% of window gain
26 people
2,600
None: people are not removed
0
Lights, computers
1,000
LED retrofit and sleep settings
400 W
The whole-system criterion: cut the total heat gain of 9,600 W by 40%. Ask each group to (1) compute the target cut, (2) compute the cut from each sub-solution, (3) add them and decide whether the recombined design meets the criterion, and (4) if it does not, say which subproblem they would reopen and why. Walk the room; the target is 3,840 W and the three sub-solutions give 2,000 + 1,200 + 400 = 3,600 W, a 240 W gap. Groups should notice that the occupant load cannot be designed away, so the gap has to come from the roof, the windows or the equipment.
Independent Practice15 minutes
Each student picks one problem from a short list: noise from the gym reaching the library, a bike rack that fills up by 8 am, or a school garden that dries out over weekends. On one page they draw a decomposition tree like Diagram 1 with at least four subproblems, write a measurable criterion and a constraint for each, circle any piece that is not an engineering problem, and mark one interface with the number the two sides must agree on. Collect the trees; they are the starting point for Homework Problem 6.
Closure5-10 minutes
Exit Ticket
"Think about the parking lot example. Give one reason the class changed the list of subproblems instead of simply making the rain garden deeper, and one check you must always do after recombining sub-solutions."
Look for two ideas: the depth limit was a constraint set for safety, so the design had to change around it; and the recombined design must be checked against the whole-system criteria and the total budget, not only piece by piece.
Differentiation Strategies
For Struggling Students
Give a decomposition template with the functions already named (collect, store, move, protect, measure) so students only fill in the criteria and constraints
Provide sentence frames for criteria: "The ___ must ___ at least/at most ___ (unit)"
Let students solve one subproblem with a partner before recombining, and give a checklist for the recombination step: total volume, total cost, every interface number
For Advanced Students
Ask students to rank the subproblems of the parking lot design by how much a 20% error in their numbers would change the whole design, and explain the ranking
Have students decompose the same problem a second way (by location on the site instead of by function) and argue which split makes the interfaces simpler
Ask students to write the interface agreement for two subteams as a one-page specification with units and tolerances
Assessment Guidance
What to Look For
Strong work names subproblems by what the system must do, not by the parts students already have in mind. Each subproblem has a number or a test attached, the budget pieces add up to no more than the total, and the student checks the recombined design against the whole-system criteria. Watch for decompositions with overlapping pieces, a missing piece that nobody owns, and "solutions" that quietly break a constraint instead of changing the plan.
02
Classroom Activities
3 Activities
1
Food Bank Card Sort
20 minGroups of 3-4
A rural county food bank loses fresh produce before it reaches families who live far from town. Groups get 12 cards, each describing one piece of the problem, and sort them into subproblems, separate the engineering pieces from the rest, and draw the interfaces.
The 12 Cards
Produce warms up in the van on trips longer than 2 hours
The warehouse cooler is full on delivery days
Nobody knows how warm the produce got during a trip
Pickup sites have no shade or cold storage
Volunteers are only free on weekends
Some families cannot reach a pickup site
Crates crush soft fruit when stacked
The van has no power outlet for a cooler
Delivery routes are planned by hand each week
Produce arrives from farms at unpredictable times
Families do not know which day produce arrives
The budget for new equipment is $5,000 this year
Procedure
Group the cards into 4-6 subproblems and give each one a short name that says what the system must do (for example "keep produce cold in transit")
Mark each subproblem E if engineering can solve it with a device, structure or process design, or N if it needs a schedule, a rule or communication. Keep the N cards on the table
For each E subproblem write one measurable criterion, such as "produce stays at or below 5 °C for a 3-hour trip"
Draw an arrow wherever one subproblem's output is another's input (the cooler in the van needs power; the crates must fit the cooler) and write the number the two sides must agree on
Discussion Questions
Which card did your group place in two subproblems at first? How did you decide where it belongs?
The equipment budget card is a constraint, not a subproblem. Which of your E subproblems will compete for it?
Which N card would make an engineering solution fail if nobody handled it?
Modification for Distance Learning
Put the 12 cards on a shared online whiteboard. Each group works in its own frame, and one person per group records the E and N labels and the interface arrows before the groups compare boards.
2
Build-and-Integrate Tower Challenge
35 minTeams of 6 (three subteams of 2)
Teams design a straw tower that is at least 60 cm tall and holds a 500 g mass on top while a "shake table" (a baking tray resting on 4 tennis balls) is moved back and forth for 10 seconds. The team splits into three subteams: foundation, frame and top platform. Each subteam solves its own piece, then the team joins the pieces and tests the whole tower.
Budget and Prices
Each team has 40 credits. Prices: 1 drinking straw = 1 credit, 30 cm of masking tape = 1 credit, 1 index card = 2 credits.
Before building, the team writes a budget split and a height split. Example plan: foundation 10 credits and 5 cm, frame 24 credits and 50 cm, top platform 6 credits and 5 cm
Example materials: foundation 4 index cards and 2 tape pieces (10 credits); frame 20 straws and 4 tape pieces (24 credits); platform 2 index cards and 1 tape piece (5 credits). This uses 39 of 40 credits and leaves 1 spare
Procedure
Team meeting (5 min): agree on the interfaces in writing: the frame's footprint on the foundation (for example a 15 cm square) and the platform's size on top of the frame
Subteams build their piece separately (15 min) and test it alone: the frame must stand 50 cm without the other parts; the platform must hold the 500 g mass while resting on a desk
Integration (5 min): join the pieces, measure the height with a meter stick and load the 500 g mass
Test (5 min): the teacher slides the tray about 5 cm left and right, twice per second, for 10 seconds while one student times it. Record pass or fail for height, load and shake
Safety: keep hands clear of the tray while it moves and use scissors only at the desks
Discussion Questions
Did any piece pass its own test but cause the whole tower to fail? Which interface was responsible?
The example plan left 1 credit spare. Where would your team spend a spare credit after the first test, and why there?
Would you split the tower into different subproblems next time, for example "resist sway" instead of "frame"?
Challenge Variation
Cut the budget to 32 credits after the first test. Teams must decide which subproblem gives up credits and predict, before rebuilding, which test is most at risk.
3
Critique a Decomposition
20 minPairs
Pairs review a student team's plan to cut the school building's electricity use by 15% in one month. They use the invented data below to find what the plan gets wrong and to rewrite the decomposition.
The Data and the Plan
Invented monthly electricity use: heating and cooling (HVAC) 42,000 kWh; lighting 18,000 kWh; computers and plug loads 12,000 kWh; kitchen 8,000 kWh; total 80,000 kWh.
Subproblem A: replace lights with LEDs, saving 40% of lighting use
Subproblem B: set computers to sleep after 10 minutes, saving 25% of plug-load use
Subproblem C: get everyone to turn off lights when they leave a room
Procedure
Compute the target saving and the saving from A and B. Does the plan reach the target?
Find the overlap: which subproblem double-counts savings another one already claims, and which one is not an engineering problem?
Find the gap: which part of the data does the plan ignore?
Rewrite the decomposition with 3-5 engineering subproblems, each with a kWh criterion, whose savings together reach the target
Discussion Questions
A and B together save 10,200 kWh against a target of 12,000 kWh. How short is the plan, and where is the missing saving most likely to come from?
HVAC is the largest load in the data, more than lighting and plug loads combined. Why might a team leave it out anyway, and why is that a problem?
How would you check, at the end of the month, whether each subproblem delivered its share?
03
Diagrams & Visual Aids
2 diagrams
Diagram 1: Decomposing the Flooding Parking Lot
A decomposition tree. The whole problem at the top carries the system-level criteria. Each box below is one subproblem with its own criterion and constraint, and the arrows between boxes are interfaces: the volume from box 1 sets the size of box 2, and so on. The dashed arrow is the recombination step, where the sub-solutions are checked together against the whole-system criteria.
Diagram 2: Where the Heat in a Portable Classroom Comes From
Invented heat gains for a portable classroom at 3 pm on a hot day, drawn to scale. Each bar is a candidate subproblem. The occupant bar (about 100 W per seated person) is shown in navy because it is a load the design cannot remove, so the other three bars have to carry the whole cut.
04
Homework Assignment
~30 min
HS-ETS1-2 Homework: Designing by Decomposition
Directions: Show every calculation with units. For each decomposition, give every subproblem a measurable criterion and a constraint. All data are invented for practice.
Part 1: Breaking It Down (Problems 1-3)
A high school of 1,200 students wants every student to be able to refill a water bottle with cold, filtered water during a 5-minute passing period. Break this problem into at least four subproblems. For each, write one measurable criterion and one constraint, and label any subproblem that engineering cannot solve.
For the refill problem, assume 30% of the students refill a 0.5 L bottle in the same passing period, and one station fills at 1.5 L per minute (ignore the time between users). (a) How many stations are needed? (b) The budget pays for only 16 stations. What flow rate per station, in L per minute, would let 16 stations serve everyone? Which subproblem does this send you back to?
A school garden irrigation system is split into four subteams: solar power, water storage, pump, and soil sensor. The pump draws 60 W and runs 30 minutes a day at 10 L per minute. The 50 W solar panel gets 4 peak-sun hours a day, and 75% of its output reaches the pump. The garden needs 250 L a day, and the tank holds 500 L. Name two interfaces between subteams, then use the numbers to check whether each subteam's choice fits the others. How many days of water does the full tank hold?
Part 2: Solving and Recombining (Problems 4-6)
A team designs a solar lantern kit for families without reliable electricity. The subproblems are: light, energy storage, energy capture and housing. The LED uses 1.5 W for 4 hours each night. Only 80% of the battery's rated capacity is usable. The site gets 5 peak-sun hours a day, and 70% of the panel's output reaches the battery. (a) How much energy does the light use each night? (b) What rated battery capacity is needed? (c) Panels come in 1 W, 2 W, 3 W and 5 W sizes. Which is the smallest panel that replaces one night's use each day?
A clinic needs an insulated carrier that keeps medicines between 2 and 8 °C for a 36-hour trip. The total carrier must weigh at most 3.0 kg and cost at most $50. The other parts (case, cold packs, thermometer) weigh 2.3 kg and cost $20. Insulation options (invented test data): foam 5 cm, 0.8 kg, 30 hours, $12; foam 7.5 cm, 1.1 kg, 42 hours, $16; vacuum panels, 0.5 kg, 60 hours, $55. (a) Check each option against all three whole-system criteria. (b) None works as the plan stands. Which other subproblem would you reopen, and what number would it need to reach so that one of the options works?
Choose the problem from your Independent Practice tree (or a new one approved by your teacher). Write a one-page design: the whole-system criteria, at least four subproblems with criteria and constraints, a budget split that adds up, one interface with its agreed number, a proposed solution for two of the subproblems, and a description of how you would test the recombined design.
Rubric
Criterion
Full Credit (2 pts)
Partial Credit (1 pt)
No Credit (0 pts)
Decomposition
Subproblems cover the whole problem without overlap; non-engineering parts identified
Most of the problem covered, or some overlap
One-piece "solution" or missing major parts
Criteria and Constraints
Every subproblem has a measurable criterion and a constraint tied to the whole problem
Criteria present but some are not measurable
Criteria missing
Calculations and Units
All sizing calculations correct with units
Correct method with one error or missing units
Calculations missing or incorrect
Recombination and Revision
Checks the combined design against every whole-system criterion and proposes a specific revision when one fails
Checks some criteria or revision is vague
No check of the whole design
05
Quiz: 20 Questions
Interactive, with answers
Instructions
Work through the questions in order. Your score updates as you answer, and Reset quiz clears everything so you or your students can try again. All data in the questions are invented for practice.
Multiple choice: pick an option to check it. Short answer: write your answer, then reveal the model answer.
0 of 20 answered · 0 correct
Question 1 of 20 · Multiple Choice
A city wants to reduce heat illness among riders waiting at bus stops in summer. What should the design team do first?
Answer: B
A complex problem is handled by first splitting it into pieces that each have a criterion and a constraint, so each piece can be designed and tested. Choice A picks a solution before the problem is defined, and choice C builds before anyone knows what the prototype must achieve. Choice D is a behavior change, not a design.
Question 2 of 20 · Multiple Choice
A team is designing a solar-powered refrigerator for a community food pantry. Which list is the most useful decomposition into engineering subproblems?
Answer: A
Choice A splits the system by function, and each piece can get a measurable criterion (watt-hours stored, a temperature, an alarm threshold). Choice B lists goals with no function or number, choice C lists purchases rather than problems, and choice D repeats one part and ignores storage, cooling and monitoring.
Question 3 of 20 · Multiple Choice
Wildfire smoke makes classroom air unhealthy for several days each fall. Which of these pieces is NOT a problem that engineering can solve with a design?
Answer: D
Moving the calendar is a policy decision. It may still matter, but it is not a device, structure or process to design, so it is kept on the list and handled by other people. Choices A, B and C are engineering subproblems: each has something to design and a number to test, such as air leakage, clean-air delivery rate or a particle reading.
Question 4 of 20 · Multiple Choice
One subproblem of the smoke design is air cleaning. A classroom is 10 m by 8 m with a 3 m ceiling. The team wants 5 air changes per hour, and each purifier delivers 340 m³ of clean air per hour. How many purifiers does this subproblem need?
Answer: C
Room volume = 10 × 8 × 3 = 240 m³. Five air changes per hour need 5 × 240 = 1,200 m³ per hour. 1,200 ÷ 340 ≈ 3.5, so 4 purifiers are needed. Choice A rounds 3.5 down, which falls short of the criterion. Choice B forgets the 5 air changes and only cleans the room volume once per hour (240 ÷ 340 rounds up to 1). Choice D assumes one purifier per air change.
Question 5 of 20 · Multiple Choice
Another subteam handles power. The classroom outlet circuit can carry 1,440 W of continuous load, and 900 W is already in use. Each purifier draws 55 W. What is the largest number of purifiers this circuit can support?
Answer: A
Power left = 1,440 - 900 = 540 W, and 540 ÷ 55 ≈ 9.8, so at most 9 purifiers fit. This is an interface number: the air-cleaning subteam must stay at or under it. Choice B rounds 9.8 up and overloads the circuit. Choice C ignores the 900 W already in use (1,440 ÷ 55), and choice D divides the load already in use by 55.
Question 6 of 20 · Multiple Choice
A school robotics rover has a 5.0 kg mass limit. Four subteams have used 1.6 kg (chassis), 1.1 kg (drive motors), 0.9 kg (battery) and 0.4 kg (camera mast). How much mass is left for the sample arm subteam?
Answer: B
Used so far: 1.6 + 1.1 + 0.9 + 0.4 = 4.0 kg, so 5.0 - 4.0 = 1.0 kg is left. Splitting a shared limit this way lets each subteam work alone. Choice C is the mass already used, not what is left. Choice D leaves out the camera mast, and choice A leaves out the battery.
Question 7 of 20 · Multiple Choice
A district is converting its school buses to electric. Which piece is still too vague to hand to one subteam as a design problem?
Answer: D
Choices A, B and C each name a function and a number the subteam can design and test against (8 hours, 4 × 19 = 76 kW, 7 m). Choice D is the kind of whole-system goal a project starts from: it has no measurable criterion yet, so it has to be broken down further, for example into liters of diesel or kilograms of carbon dioxide saved per year, before anyone can design for it.
Question 8 of 20 · Multiple Choice
A campus delivery robot must run at least 60 minutes, and its power and drive systems together may weigh at most 2.2 kg. Invented test data: Power P1 weighs 0.9 kg and runs 50 min; power P2 weighs 1.4 kg and runs 80 min. Drive D1 weighs 0.9 kg; drive D2 weighs 0.6 kg. Which combination meets both whole-system criteria?
Answer: C
P1 runs only 50 minutes, so choices A and B fail the run-time criterion no matter which drive is used. P2 with D1 weighs 1.4 + 0.9 = 2.3 kg, over the 2.2 kg limit. P2 with D2 weighs 1.4 + 0.6 = 2.0 kg and runs 80 minutes, so it meets both. The best sub-solutions only show up when the pieces are checked together.
Question 9 of 20 · Multiple Choice
In a design for a school greenhouse, the heating subteam finds a heater that meets its temperature criterion but uses 70% of the total project budget. What is the best next step?
Answer: A
A sub-solution that meets its own criterion can still break a whole-system constraint. The team has to look at the allocation and decide on trade-offs together. Choices B and D treat the pieces as independent, which hides the conflict until recombination, and choice C removes a required function.
Question 10 of 20 · Multiple Choice
Every subteam's part of a water-filter kit passed its own test, but the assembled kit leaks. What is the most likely cause?
Answer: D
When every piece works alone and the whole fails, the failure usually sits where the pieces meet. That is why interfaces must be written down with numbers before the subteams design. Choice A contradicts the question: every part passed. Choices B and C reject decomposition instead of fixing the interface.
Question 11 of 20 · Multiple Choice
An emergency shelter must store drinking and cooking water for 150 people for 3 days. The team plans 4 L per person per day, and the storage subproblem uses 500 L tanks. How many tanks are needed?
Answer: B
Water needed = 150 × 3 × 4 = 1,800 L. 1,800 ÷ 500 = 3.6, so 4 tanks are needed. Choice A rounds 3.6 down and leaves the shelter 300 L short. Choice C plans for only one day (600 L ÷ 500 L, rounded up), and choice D divides the 1,800 L by the 150 people instead of by the tank size.
Question 12 of 20 · Multiple Choice
Which criterion is written well enough for a subteam to design and test against?
Answer: C
Choice C names a condition, a number and a test, so the subteam knows exactly when it has succeeded. Choices A, B and D are wishes with no measurement: two teams could read them in opposite ways.
Question 13 of 20 · Multiple Choice
A hospital designs a backup power system. Its subproblems are keeping life-support equipment running, keeping the lights on in hallways, keeping the cafeteria kitchen open, and limiting generator noise. If the budget cannot cover all four, which criterion should keep top priority?
Answer: A
Trade-offs are decided by priority, and safety to human life comes before comfort or convenience. The other criteria still matter and can be met with the remaining budget, but choices B, C and D should give way first if the budget forces a choice.
Question 14 of 20 · Multiple Choice
A community garden plans a rooftop rain-collection system with these subproblems: size the storage tank, design the gutters, choose a pump, and estimate how much water the garden uses each week. Which subproblem should be solved first?
Answer: D
The tank size and the pump both depend on the demand, so the demand estimate is an input to the other pieces. Solving the tank first (choice C) means guessing its most important number. Choices A and B also depend on flows that come from the demand and the roof.
Question 15 of 20 · Short Answer
The crosswalk outside a school is busy and drivers often do not stop. Break the problem "make the crosswalk safer for students" into at least three engineering subproblems with one measurable criterion each, and name one part of the problem that is not an engineering problem.
Sample answer. Visibility: drivers can see a student at the curb from at least 60 m away (lighting, flashing beacons, trimmed hedges). Speed: car speeds near the crossing drop to 25 mph or less (speed table, narrower lane). Crossing distance: students are in the road for no more than 8 seconds (curb extensions, a median refuge). Not engineering: adult crossing guard schedules, or a rule about where parents drop off. Any set of subproblems that splits the problem by function with a number in each criterion earns full credit.
Question 16 of 20 · Short Answer
A hand-cranked phone charger for emergency kits is split into crank, generator, storage and connector subproblems. The generator gives 5 W while cranked, 60% of that energy reaches storage, and the criterion is 1 Wh stored for an emergency call. How long must someone crank? The whole-system criterion says no more than 10 minutes. Which subproblem must be reopened, and what generator output would meet it at the same efficiency?
Energy stored per hour of cranking = 5 W × 0.6 = 3 W, so 1 Wh takes 1 ÷ 3 h = 1/3 h = 20 minutes. That fails the 10-minute criterion. Reopen the generator subproblem (or the storage efficiency): at 60% efficiency the generator must give 10 W, because 1 Wh ÷ (10 W × 0.6) = 1/6 h = 10 minutes.
Question 17 of 20 · Short Answer
A student team wants to reduce litter on campus. Their subproblems are: (1) put out more trash cans, (2) choose bins that crows and raccoons cannot open, (3) put more bins near the cafeteria. Name one weakness of this decomposition and rewrite it.
Subproblems 1 and 3 overlap: both are "more bins", so two people could work on the same thing while other parts go missing. None of the pieces has a measurable criterion, and nothing handles emptying the bins before they overflow or finding where litter actually comes from. A better split: measure litter by location each week (the baseline); capacity and placement: no bin more than 30 m from any lunch table and no bin over full at the end of lunch; animal-proof lids that stay closed when pushed; collection: bins emptied before they reach 90% full. A rule or a campaign about littering is a non-engineering part.
Question 18 of 20 · Short Answer
Two subteams are building a bike light powered by a generator in the wheel hub. Write the interface agreement they need before either subteam starts designing. Include at least three items with units.
Sample answer: the voltage the generator delivers to the light (for example 6 V), the maximum power the light may draw (for example 3 W, so the generator can supply it at normal riding speed), the connector type and wire length (for example a 2-pin plug on a 60 cm lead), and what happens when the bike stops (for example the light stays on for 4 minutes from a small storage part). Any three specific items with units that both teams must match earn credit.
Question 19 of 20 · Short Answer
A portable handwashing station for outdoor events is split into four subteams. Invented results: water tank 40 L (3 kg empty), foot pump (2 kg), frame (6 kg), greywater tank 30 L (2 kg). Whole-system criteria: empty mass at most 15 kg so two people can carry it, at least 200 handwashes per fill at 0.2 L per wash, and no overflow. Does the recombined design meet every criterion? If not, which interface failed?
Empty mass = 3 + 2 + 6 + 2 = 13 kg, which is within 15 kg. Handwashes per fill = 40 ÷ 0.2 = 200, which meets the criterion. But the greywater tank holds only 30 L while a full fill sends 40 L of used water into it, so it overflows by 10 L. The failed interface is between the water tank and the greywater tank: their volumes were never matched. A greywater tank of at least 40 L fixes it (check that its extra mass keeps the station within 15 kg).
Question 20 of 20 · Short Answer
In two or three sentences, explain why engineers break complex problems into smaller ones, and describe one risk that decomposition creates.
Smaller problems can each be given clear criteria, designed, built and tested by a subteam, which makes a problem that is too big to solve at once workable and lets teams work in parallel. The risk is that the pieces are solved separately and then fail to fit or together miss a whole-system criterion, for example two parts that together go over budget. That is why interfaces must be agreed on early and the recombined design must be tested as a whole.
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06
Frequently Asked Questions
10 Questions
What does HS-ETS1-2 mean?
HS-ETS1-2 asks high school students to design a solution to a complex real-world problem by breaking it into smaller problems that engineering can solve.
Students do not just list parts: they give each subproblem its own criteria and constraints, solve those pieces, and show that the pieces work together as one design. The code reads: HS (high school), ETS1 (Engineering Design), performance expectation 2.
Is HS-ETS1-2 taught in physics, chemistry or biology?
It can be taught in any of them, because engineering design is meant to be woven into the science courses.
Physics classes often use it with energy or force projects, biology classes with environmental problems, and chemistry classes with water treatment or materials. Some schools also teach it in a separate engineering course.
How is HS-ETS1-2 different from HS-ETS1-1 and HS-ETS1-3?
HS-ETS1-1 defines the problem, HS-ETS1-2 breaks it down and designs the solution, and HS-ETS1-3 evaluates solutions against prioritized criteria and trade-offs.
In practice they overlap in one project. HS-ETS1-4 adds a computer simulation to test how a solution behaves across a whole system.
What counts as a "complex real-world problem"?
A problem with several interacting parts, several criteria at once and shared limits on money, space, time or energy.
Good classroom examples are local and concrete: a flooding lot, an overheated room, food waste, a busy crosswalk. A problem with one part and one criterion, such as "build a taller tower", is a design task but not a complex problem in the sense of this standard.
How small should the subproblems be?
Small enough that each one has a measurable criterion and can be designed and tested on its own.
If students cannot write a number or a test for a piece, it is still too big or too vague. If a piece is only "buy part X", it has been split past the point of being a design problem.
Do students have to build something to meet HS-ETS1-2?
No. The performance expectation asks students to design a solution, which can be shown with drawings, calculations and a written plan.
Building a prototype, as in the tower challenge, helps students see interface failures, but a well-argued design with sizing calculations for each subproblem also meets the standard.
What mistakes do students make when they decompose a problem?
Frequent ones are subproblems that overlap, a missing piece that nobody owns, and criteria with no number.
Another is solving each piece and never checking the whole: every part meets its own criterion, but together they go over budget or do not fit. Ask for a recombination check in every task.
How is HS-ETS1-2 assessed?
Through tasks where students decompose a problem and justify a design, not through recall of vocabulary.
State science tests aligned to NGSS assess engineering performance expectations in different ways, so check your state's test design documents. In class, the rubric in the homework section scores the decomposition, the criteria, the calculations and the recombination.
Where does math come in?
In sizing each subproblem: areas and volumes, rates, energy in watt-hours, percentages and budget sums.
The calculations are usually short, but they turn a vague piece ("store the water") into a solvable one ("store 45 m³"). They also connect to the Common Core modeling standards, such as choosing units and quantities.
Can HS-ETS1-2 be taught online?
Yes. Decomposition trees, interface agreements and sizing calculations work well on a shared whiteboard or document.
For a hands-on part at home, students can build the tower challenge with household straws and tape, or evaluate a local problem they can observe, such as drainage on their street, and share photos as evidence.
07
Related Standards
6 standards
These standards connect to HS-ETS1-2: prerequisites to review first, parallel standards at the same level, and next steps that build on it.
Before this lesson
MS-ETS1-1Prerequisite
Define the criteria and constraints of a design problem precisely enough to succeed
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