HS-PS2-3: Designing a Device That Reduces Collision Forces
In plain English: HS-PS2-3 is the NGSS high school performance expectation that asks students to design, evaluate and refine a device that reduces the force on an object during a collision. Students use the idea that average force equals change in momentum divided by stopping time to judge helmets, padding and parachutes against criteria. It is usually taught in Physics or Physical Science.
Apply scientific and engineering ideas to design, evaluate, and refine a device that minimizes the force on a macroscopic object during a collision.
Clarification Statement: Examples of evaluation and refinement could include determining the success of a device at protecting an object from damage and modifying the design to improve it. Examples of a device could include a football helmet or a parachute.
Assessment Boundary: Assessment is limited to qualitative evaluations and/or algebraic manipulations.
Next Generation Science Standards · Disciplinary Core Idea: Motion and Stability: Forces and Interactions · Official standard
Students apply one physics idea to an engineering problem: when an object stops, its change in momentum is fixed by its mass and speed, so the average force on it is F = Δp/Δt = mΔv/Δt. A device that makes the stop last longer, or that spreads it over a longer crush distance, lowers the force. Students design a protective device, test it, evaluate it against criteria and constraints with evidence, and refine it. The devices include an egg-drop package, helmet padding and a parachute.
The NGSS Clarification Statement says: "Examples of evaluation and refinement could include determining the success of a device at protecting an object from damage and modifying the design to improve it. Examples of a device could include a football helmet or a parachute." The Assessment Boundary says: "Assessment is limited to qualitative evaluations and/or algebraic manipulations." Every calculation on this page is an algebraic rearrangement of F = mΔv/Δt or of the constant-force stopping relation F = mv²/(2d); there are no calculus-based force curves. All test data are invented for teaching. Students do the practice of designing solutions: they set criteria, build and test, compare designs with numbers, and justify a refinement.
Learning Objectives
By the end of this lesson, students will be able to:
Explain why increasing the stopping time or stopping distance reduces the average force on an object whose change in momentum is fixed
Use F = mΔv/Δt and F = mv²/(2d) to calculate average forces and to solve for the stopping time or distance a design needs
Design a device that protects an object in a collision, stating criteria and constraints before building
Evaluate one or more designs against criteria and constraints using test data, not opinion
Refine a design by changing one feature and predicting, with a calculation or a qualitative argument, how the force will change
Prior Knowledge Required
Students should already be comfortable with:
Newton's second law and net force HS-PS2-1
Momentum, p = mv, and its conservation in collisions HS-PS2-2
Criteria and constraints in an engineering design problem MS-ETS1-1
Rearranging a formula to solve for one variable HSA.CED.A.4
Toss a raw egg (sealed in a zip-top bag) gently into a bedsheet held loosely by two students, then show a slow-motion video of a hard-boiled egg dropped on a table. Ask:
Warm-Up Prompt
"The egg hits the sheet and the table at about the same speed, and it stops both times. What is the same about the two stops, and what is different? Why does one egg survive?"
Collect ideas. Students often say the sheet is "soft"; push them to describe what soft does (the stop takes longer and happens over a longer distance). Write "same: change in speed; different: stopping time" on the board to return to.
Direct Instruction15-20 minutes
The physics a protective device uses. Newton's second law can be written as F = Δp/Δt: the average net force equals the change in momentum divided by the time the change takes. When an object moving at speed v stops, Δp = mv no matter what it hits. So the only ways a device can lower the average force are to make the stop last longer (larger Δt) or to lower the speed before impact (smaller Δp). For a stop with constant force over a crush distance d, the same idea becomes F = mv²/(2d): doubling the crush distance halves the force. For quick stops the object's weight is small compared with these forces, so we treat the average net force as the force from the surface. Use Diagram 1 to show two stops with the same area under the force-time graph.
Longer stopping time
A 0.060 kg egg hits the floor at 6.0 m/s. On a hard floor it stops in 0.0020 s; on a foam pad it stops in 0.030 s.
Result: Δp = 0.060 × 6.0 = 0.36 kg·m/s both times. Hard floor: F = 0.36 ÷ 0.0020 = 180 N. Foam: F = 0.36 ÷ 0.030 = 12 N, fifteen times smaller.
Helmet padding (football helmet)
In a test, a 5.0 kg headform moving at 4.0 m/s stops against a hard surface. Without padding it stops in 0.0050 s; with the helmet's foam liner it stops in 0.020 s.
Result: Δp = 20 kg·m/s. Without liner: 20 ÷ 0.0050 = 4,000 N. With liner: 20 ÷ 0.020 = 1,000 N. The liner does not change Δp; it stretches the stop over four times as long.
Longer crush distance
A 1.2 kg lab cart moving at 2.0 m/s hits a barrier. A stiff bumper crushes 0.50 cm; a foam bumper crushes 3.0 cm.
Result: F = mv²/(2d). Stiff: 1.2 × 4.0 ÷ (2 × 0.0050) = 480 N. Foam: 1.2 × 4.0 ÷ (2 × 0.030) = 80 N. Six times the crush distance gives one sixth of the force.
Lower impact speed (parachute)
A 0.20 kg payload lands at 7.0 m/s without a parachute and at 2.0 m/s with one. Either way the ground stops it in about 0.010 s.
Result: Without: 0.20 × 7.0 ÷ 0.010 = 140 N. With: 0.20 × 2.0 ÷ 0.010 = 40 N. The parachute works on Δp, not on Δt: it lowers the speed before the collision.
Evaluate, then refine
A 0.18 kg phone reaches the floor at 5.0 m/s. The criterion is an average force of 150 N or less. Case A stops the phone in 0.0030 s; case B in 0.0090 s.
Result: Case A: 0.90 ÷ 0.0030 = 300 N (fails). Case B: 0.90 ÷ 0.0090 = 100 N (passes). To refine case A, solve Δt = mΔv/F = 0.90 ÷ 150 = 0.0060 s: its corners must at least double the stopping time.
After each example, ask: "Which quantity did the design change: the change in momentum or the stopping time? What would you measure to check it?" Introduce the design cycle in Diagram 2 and name the criteria (what success means) and constraints (limits on mass, cost, size or materials) before any building starts.
Guided Practice15-20 minutes
Pairs evaluate invented helmet-pad test data. In each test a 2.0 kg headform hits the pad at 3.0 m/s. The criterion is an average force of 500 N or less, and the constraint is that the liner can be at most 3.0 cm thick.
Guided practice data: helmet pad tests
Pad
Thickness (cm)
Stopping time (s)
Bare shell
0
0.0030
Thin foam
1.0
0.0080
Thick foam
2.5
0.015
Foam with air cells
4.0
0.024
Find the change in momentum of the headform (6.0 kg·m/s) and explain why it is the same for every pad.
Compute the average force for each pad: 2,000 N, 750 N, 400 N and 250 N.
Evaluate: which pads meet the force criterion, and which of those also meet the thickness constraint? (Only the thick foam meets both.)
Refine: propose one change to the foam with air cells that keeps its low force but meets the constraint, and say what you would measure to test it.
Independent Practice10-15 minutes
Each student designs packaging on paper. A 0.40 kg glass jar may be dropped during shipping and hit the floor at 4.0 m/s. The jar breaks if the average force on it goes above 200 N. Students find the shortest stopping time the packaging must give (0.40 × 4.0 ÷ 200 = 0.0080 s) and, assuming a constant force, the crush distance that goes with it (d = vΔt/2 = 1.6 cm). Then they sketch a design, list its criteria and constraints, and explain in two or three sentences how the design lengthens the stop.
Closure5 minutes
Exit ticket: "A new pad doubles the stopping time of the same object at the same speed. What happens to the change in momentum, and what happens to the average force? Name one design feature that could double the stopping time." (The change in momentum stays the same; the force is halved.) Return to the warm-up and have students label the bedsheet and the table with "long Δt" and "short Δt".
Differentiation Strategies
For Struggling Students
Give a three-column table (Δp, Δt, F = Δp ÷ Δt) and have students fill it in for each design before comparing
Use one fixed object and speed for all calculations so that only the stopping time changes at first
Provide a sentence frame for evaluations: "Design ___ meets / does not meet the criterion because its force is ___ N, compared with the limit of ___ N."
For Advanced Students
Ask students to show that F = mv²/(2d) follows from F = ma and v² = 2ad, and to explain when the constant-force assumption underestimates the peak force
Have students build a weighted decision matrix that combines force, mass, cost and size, and defend the weights they chose
Ask students to explain qualitatively why a pad that makes the object bounce back gives a larger force than one that stops it, for the same contact time
Assessment Guidance
What to Look For
Check that students keep Δp fixed when comparing pads for the same object and speed, and that they state which quantity a device changes (stopping time, crush distance or impact speed). Look for evaluations that compare a measured or calculated value with a stated criterion and mention the constraints, rather than "it worked" or "it looked strong". Listen for students who think a stronger, stiffer container protects better, or who think padding reduces the object's momentum instead of the force.
02
Classroom Activities
3 Activities
1
Egg Drop: Design, Test, Refine
40 minGroups of 3
Teams design a package that keeps a raw egg from cracking when dropped from 2.0 m. They write criteria and constraints first, build, test, estimate the stopping time from slow-motion video, and refine the design once. Ignoring air resistance, the package hits the floor at about √(2 × 9.8 × 2.0) = 6.3 m/s.
Criteria and Constraints
Criterion: the egg is uncracked after a 2.0 m drop, in two trials in a row.
Constraints: at most 100 g of materials (not counting the egg), only the materials provided (straws, paper, tape, cotton balls, rubber bands, plastic bags), 15 minutes of build time, and no parachutes in round 1.
Procedure and Safety
Seal each egg in a zip-top bag before it goes into the device, and wash hands after handling raw eggs. Check for egg allergies first.
The teacher drops each package from a marked 2.0 m height while standing on a stable platform or stair landing with a spotter; everyone else stays out of the drop zone and wears safety goggles.
Film each landing in slow motion next to a meter stick and count frames from first contact to stop.
Sample Data (invented)
One team's first design, a straw frame with the egg taped in the center, cracked; video showed the egg stopped in about 0.004 s. Their refined design added a crushable paper cone under the egg; the egg stopped in about 0.020 s and survived both trials. For a 0.060 kg egg at 6.3 m/s, the average force estimates are about 94 N and 19 N.
Discussion Questions
The refinement made the stop about five times longer. By what factor did the estimated force drop, and why?
Which design feature produced the longer stop? Would a stiffer frame have helped?
If you could relax one constraint, which would help most, and what would you change?
2
Helmet Padding Test
25 minPairs
Modeled on how football helmet liners are compared, pairs drop a 0.50 kg sandbag from 0.50 m onto four pad materials and measure how far each pad compresses using slow-motion video against a ruler. With F = mv²/(2d) and v² = 2gh, the average force is F = mgh/d.
Procedure
Tape a ruler upright beside the pad and film from the side at 240 frames per second if the phone allows it.
Drop the sandbag from 0.50 m three times per pad and record the largest compression each time; average the three.
Safety: keep hands clear of the landing area and use a sandbag, not a heavy metal mass.
Sample Data (invented)
Average compression: packing foam 0.8 cm, folded towel 1.2 cm, bubble wrap 1.6 cm, pool noodle slice 2.0 cm. With mgh = 0.50 × 9.8 × 0.50 = 2.45 J, the average forces are about 306 N, 204 N, 153 N and 122.5 N.
Discussion Questions
Going from packing foam to bubble wrap doubles the compression. What happens to the average force, and does the data show it?
Which pad gives the lowest force, and what might make it a poor choice for a real helmet anyway (think of constraints)?
The formula gives an average. Why would the peak force during a real impact be higher?
Modification for Distance Learning
Students test household materials under a dropped bag of rice at home, or use the sample data above, and post a ranked recommendation with their calculations to the class discussion board.
3
Parachute Refinement Challenge
30 minGroups of 3-4
Teams build parachutes from plastic bags and string to land a 10 g payload of washers from 2.0 m. They estimate the landing speed from the fall time, compute the landing force for a 0.010 s stop, evaluate against the criteria and refine the canopy.
Criteria and Constraints
Criterion: estimated landing force of 1.2 N or less.
Constraint: the canopy fits inside a 35 cm by 35 cm square of plastic.
Safety: the drop is made from a stable step stool with a spotter, never from a chair or table.
Sample Data (invented)
Square canopies of 20, 30 and 40 cm fell 2.0 m in 1.1 s, 1.6 s and 2.0 s. Because the payload reaches a nearly steady speed early in the fall, the average speed approximates the landing speed: about 1.8, 1.25 and 1.0 m/s. For a 0.010 kg payload stopping in 0.010 s, the landing forces are about 1.8 N, 1.25 N and 1.0 N. Without a parachute the payload would land at about 6.3 m/s, giving about 6.3 N.
Discussion Questions
None of the three canopies meets both the force criterion and the size constraint. Which one comes closest, and what refinement could close the gap?
How many times smaller is the landing force with the 40 cm canopy than with no parachute?
Why is the average speed a slight underestimate of the landing speed?
Challenge Variation
Keep the 30 cm canopy and change its shape (dome, vent hole or longer strings). Predict whether the fall time will go up or down, test three drops, and report whether the refinement meets the criterion.
03
Diagrams & Visual Aids
2 diagrams
Diagram 1: Same Change in Momentum, Different Stopping Times
Idealized force-time graphs, drawn to scale, for a 0.060 kg egg that hits at 6.0 m/s. Each triangle has an area of 0.36 N·s, the egg's change in momentum. The hard floor stops the egg in 0.002 s with an average force of 180 N; the foam pad takes 0.030 s, so the average force is only 12 N.
Diagram 2: The Design Cycle for a Protective Device
Students define criteria and constraints once, then loop through design, test, evaluate and refine. Evaluation uses data: the calculated average force is compared with the criterion, and the design is checked against every constraint.
04
Homework Assignment
~30 min
HS-PS2-3 Homework: Designing for Smaller Collision Forces
Directions: All test data are invented for practice. Use F = mΔv/Δt or F = mv²/(2d) for average forces, show each step with units, and write every evaluation as a comparison with a stated criterion or constraint.
Part 1: Calculating and Comparing Forces (Problems 1-2)
In a crash test, a 70 kg driver moving at 15 m/s is brought to rest. (a) Find the driver's change in momentum. (b) Find the average force if an airbag stops the driver in 0.12 s. (c) Find the average force if the driver's head instead hits the dashboard and stops in 0.015 s. (d) How many times larger is the dashboard force? Explain what the airbag changes.
A bike helmet's foam crushes 2.5 cm when a 4.5 kg head moving at 5.0 m/s stops against it. (a) Find the average force, assuming it is constant. (b) The designers want to lower the average force to 1,500 N for the same impact. How thick a crush zone does the foam need? (c) Name one constraint that limits how thick the foam can be.
Part 2: Evaluating Designs with Data (Problems 3-4)
Three phone-case prototypes are tested with a 0.20 kg phone that hits the floor at 4.4 m/s. Stopping times: case X 0.0020 s, case Y 0.0044 s, case Z 0.0055 s. Case masses: X 20 g, Y 32 g, Z 48 g. The criterion is an average force below 250 N and the constraint is a case mass of 40 g or less. (a) Find the average force for each case. (b) Which cases meet the criterion, and which meet both the criterion and the constraint? (c) Propose one refinement to case Z and explain what it must keep the same.
A 50 kg box of relief supplies is dropped by parachute and lands on a cardboard crush pad that compresses 0.20 m. (a) Find the average net stopping force if it lands at 8.0 m/s. (b) Find it again for a larger parachute that gives a landing speed of 5.0 m/s. (c) By what factor did the force drop? Explain why the factor is larger than 8.0 ÷ 5.0. (d) Give one trade-off of using the larger parachute.
Part 3: Designing and Refining (Problems 5-6)
Design a knee pad for skateboarders. In a fall, the knee and lower leg (about 6.0 kg) hit the ground at 3.0 m/s, and bare skin on concrete stops them in about 0.0040 s. (a) Find the average force without a pad. (b) Your criterion is 1,000 N or less. Find the stopping time the pad must give. (c) Assuming a constant force, find the crush distance that goes with that time. (d) Sketch or describe your pad, list two constraints, and describe how you would test it.
A student's egg-drop design is a rigid plastic box with the egg taped tightly inside and no padding. The student says, "The box is so strong that it will never break, so the egg will be safe." (a) Evaluate the claim using the idea of stopping time. (b) Predict what the test will show. (c) Propose two refinements and explain how each one lowers the force on the egg.
Rubric
Criterion
Full Credit (2 pts)
Partial Credit (1 pt)
Force Calculations
Correct Δp, stopping time or crush distance, and average force, with units and correct rearranging of the formula
Correct method with one arithmetic, unit or rearranging error
Evaluation Against Criteria
Compares each design's value with the stated criterion and constraint and names which designs pass
Compares with the criterion but ignores a constraint, or the reverse
Design and Refinement
Proposes a specific change and explains, with physics, whether it lengthens the stop or lowers the impact speed
Proposes a change without a physics reason
05
Quiz: 20 Questions
Interactive, with answers
Instructions
All test data are invented. Use F = mΔv/Δt or F = mv²/(2d) for average forces. Your score updates as you answer, and Reset quiz clears everything so you or your students can try again.
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
Why does an airbag reduce the force on a driver in a crash?
Answer: B
The driver's change in momentum is fixed by mass and speed, so the airbag lowers F = Δp/Δt by making Δt larger. Choice A is a common misconception: the driver still goes from full speed to rest. Choice C would make the force larger, not smaller.
Question 2 of 20 · Multiple Choice
A 0.50 kg ball moving at 9.0 m/s is caught and stopped in 0.050 s. What is the average force on the ball?
Answer: D
Δp = 0.50 × 9.0 = 4.5 kg·m/s, and F = 4.5 ÷ 0.050 = 90 N. Choice A is the change in momentum, not the force. Choice B multiplies Δp by the time instead of dividing. Choice C divides by 0.50 s, a decimal-place error.
Question 3 of 20 · Multiple Choice
A new pad triples the stopping time of an object that hits it at the same speed. How does the average force change?
Answer: A
F = Δp/Δt with Δp unchanged, so tripling Δt divides the force by 3. Choice B reverses the relationship. Choice D squares the factor; that would be true only if the impact speed were also cut to one third.
Question 4 of 20 · Multiple Choice
The same phone is dropped from the same height onto a tile floor and onto thick carpet. Which quantity is the same for both landings?
Answer: C
The phone goes from the same speed to rest both times, so Δp = mv is the same (and so is the area under the force-time graph). Choices A and D are what the carpet changes, and because they change, the average force (choice B) is different.
Question 5 of 20 · Multiple Choice
A team is designing an egg-drop package. Which statement is a constraint rather than a criterion?
Answer: B
A constraint is a limit on the design (mass, cost, size, materials, time); a criterion describes what success looks like. Choices A, C and D all describe success, so they are criteria. Only the cost limit in choice B restricts how the team may build.
Question 6 of 20 · Multiple Choice
Assuming a constant stopping force, what happens to the average force if a bumper's crush distance is cut in half for the same object and speed?
Answer: B
F = mv²/(2d), so with m and v fixed the force is inversely proportional to d: halving d doubles F. Choice A reverses the relationship. Choice C squares the factor, as if F depended on 1/d²; a factor of four would need the impact speed to double instead.
Question 7 of 20 · Multiple Choice
How does a parachute mainly reduce the force on a payload when it lands?
Answer: D
Air resistance on the canopy lowers the landing speed, so Δp = mv at impact is smaller, and for the same stopping time the force is smaller. Choice B describes padding, not a parachute: the ground stops the payload just as quickly. Choice C is false: gravity still acts on the payload.
Question 8 of 20 · Multiple Choice
A 1.5 kg object hits a pad at 4.0 m/s. The criterion is an average force of 300 N or less. Which of these stopping times meets the criterion?
Answer: C
Δp = 6.0 kg·m/s, so the force is 300 N or less only if Δt ≥ 6.0 ÷ 300 = 0.020 s. Of the choices, only 0.025 s works (240 N). Choice B gives 400 N and choice A gives 600 N: both fail. Choice D confuses a shorter time with a gentler stop.
Question 9 of 20 · Multiple Choice
A football helmet has a hard outer shell and a soft inner liner. What does each part contribute to protecting the head?
Answer: B
The rigid shell spreads the impact over a larger area and resists puncture, while the crushable liner makes the head's stop last longer, which lowers the average force. Choice A repeats the misconception that padding lowers Δp. Choice C would raise the force.
Question 10 of 20 · Multiple Choice
A team's egg cracked in testing. Which refinement most directly lowers the average force on the egg for the same drop?
Answer: A
A crushable layer lengthens the stopping distance and time, so F = Δp/Δt drops. Choices B and D make the stop more sudden: a stiffer container passes the full impact to the egg quickly. Choice C has no effect on force.
Question 11 of 20 · Multiple Choice
A 0.10 kg object will hit the ground at 4.0 m/s. The force on it must stay at or below 25 N. What is the shortest stopping time the device can allow?
Answer: D
Δt = mΔv/F = (0.10 × 4.0) ÷ 25 = 0.40 ÷ 25 = 0.016 s. Choice A mistakes the change in momentum (0.40 kg·m/s) for the time. Choice B divides the force by the change in momentum, inverting the ratio. Choice C multiplies the force by the change in momentum.
Question 12 of 20 · Multiple Choice
Why should a team test each version of a protective device several times before evaluating it?
Answer: C
Drops vary a little in height, angle and landing spot, and video timing has uncertainty. Several trials show whether a result is repeatable and give a more reliable average for comparing designs. Choice B overstates the problem: single trials are not always wrong, just less reliable.
Question 13 of 20 · Multiple Choice
A 0.20 kg ball moving at 5.0 m/s hits a wall. Pad 1 stops it; pad 2 sends it back at 3.0 m/s. What is the size of the ball's change in momentum with pad 2?
Answer: B
Taking the first direction as positive, Δp = 0.20 × (-3.0 - 5.0) = -1.6 kg·m/s, size 1.6 kg·m/s. Choice A subtracts the speeds without signs (5.0 - 3.0). Choice C is the change for pad 1, which only stops the ball. For the same contact time, the bouncy pad gives the larger force.
Question 14 of 20 · Multiple Choice
Which evaluation of two helmet designs is based on evidence?
Answer: A
An engineering evaluation compares measured or calculated results with the criteria and constraints. Choice A cites repeated test results and a constraint. Choices B, C and D are opinions or effort, not evidence about how well the device protects the head.
Question 15 of 20 · Short Answer
A 60 kg gymnast lands at 5.0 m/s. Find the average net force on the gymnast if (a) a mat stops her in 0.25 s and (b) a hard floor stops her in 0.050 s.
Δp = 60 × 5.0 = 300 kg·m/s. (a) F = 300 ÷ 0.25 = 1,200 N. (b) F = 300 ÷ 0.050 = 6,000 N. The mat makes the stop five times longer, so the force is five times smaller.
Question 16 of 20 · Short Answer
A 1.2 kg delivery drone may hit the ground at 6.0 m/s if its motors fail. Its landing legs must keep the average force at or below 360 N. Assuming a constant force, what is the smallest crush distance the legs must provide?
Rearrange F = mv²/(2d): d = mv²/(2F) = (1.2 × 36) ÷ (2 × 360) = 43.2 ÷ 720 = 0.060 m, or 6.0 cm.
Question 17 of 20 · Short Answer
Two bike-helmet designs were tested. Design P: average force 900 N, mass 450 g. Design Q: average force 700 N, mass 650 g. The criterion is 1,000 N or less, and the constraint is a mass of 500 g or less. Which design do you recommend, and what refinement would you try next?
Both designs meet the force criterion (900 N and 700 N are below 1,000 N), but only design P meets the mass constraint. Recommend P. A good next refinement is to make P's liner crush more gradually (for example, a softer or thicker foam where the mass allows it) to lower the force further, or to find what gives Q its low force and add it to P without exceeding 500 g.
Question 18 of 20 · Short Answer
Plan a fair test to compare two new padding materials for a helmet. Name what you would keep the same, what you would change, what you would measure, and how you would turn the measurement into a force.
Keep the same drop mass, drop height (so the same impact speed), pad area and thickness, and landing surface; change only the padding material. Measure the stopping time or the crush distance with slow-motion video next to a ruler, in at least three trials per material. Compute F = mΔv/Δt or F = mv²/(2d) and compare the averages with the force criterion.
Question 19 of 20 · Short Answer
A 0.10 kg payload lands at 3.0 m/s with a small parachute and at 1.5 m/s with a larger one. In both cases the ground stops it in 0.020 s. Find both landing forces and explain which quantity the larger parachute changed.
Small parachute: 0.10 × 3.0 ÷ 0.020 = 15 N. Larger parachute: 0.10 × 1.5 ÷ 0.020 = 7.5 N. The stopping time is the same, so the larger parachute changed the change in momentum at landing by halving the impact speed.
Question 20 of 20 · Short Answer
Cars have crumple zones that are designed to be crushed in a crash. Explain qualitatively how a crumple zone protects the passengers, even though it makes the car more damaged.
The passengers' change in momentum is set by their mass and speed. As the crumple zone crushes, it lengthens the time and distance over which the passenger compartment stops, so the average force on the passengers, F = Δp/Δt, is smaller. The crushed metal is the part of the device that is sacrificed so the people inside are not.
0 of 20 answered · 0 correct
06
Frequently Asked Questions
10 Questions
What does HS-PS2-3 mean?
HS-PS2-3 asks students to use science and engineering ideas to design, evaluate and refine a device that reduces the force on an object in a collision. The key idea is that the average force equals the change in momentum divided by the stopping time, so devices work by making the stop last longer or by lowering the impact speed. The code means High School, Physical Science, core idea 2 (Motion and Stability: Forces and Interactions), performance expectation 3.
What is the assessment boundary for HS-PS2-3?
Assessment is limited to qualitative evaluations and algebraic manipulations. Students may rearrange F = Δp/Δt or F = mv²/(2d) and compare designs in words. Integrating a changing force over time with calculus, or modeling a detailed force curve, is beyond this standard.
What devices can students design for HS-PS2-3?
Any device that protects an object during a collision works. The Clarification Statement names a football helmet and a parachute. Classroom versions include egg-drop packages, helmet pads tested with a dropped sandbag, cart bumpers, phone cases and small plastic-bag parachutes.
Why does increasing the stopping time reduce the force?
Because the change in momentum is fixed, and force is that change divided by the time it takes. An object moving at a given speed must lose all its momentum when it stops. If the stop takes ten times as long, the average force is one tenth as large. On a force-time graph the area stays the same while the graph becomes lower and wider.
What is the difference between criteria and constraints?
Criteria describe what counts as success; constraints are the limits the design must stay within. For an egg drop, "the egg does not crack" is a criterion, while "no more than 100 g of materials" is a constraint. Good evaluations check a design against both.
How do students evaluate a design with evidence?
They compare test results with the stated criteria. That means measuring something (stopping time from slow-motion video, crush distance against a ruler, landing speed from fall time), calculating the average force, and checking the design's mass, size and cost against the constraints. Several trials per design make the comparison more reliable.
Is HS-PS2-3 a physics standard or an engineering standard?
Both: it is a physical science performance expectation that uses an engineering practice. The science is Newton's second law in momentum form; the engineering is designing, testing and refining a solution against criteria and constraints. It pairs naturally with the HS-ETS1 engineering design standards.
What are common mistakes students make with HS-PS2-3?
A common one is thinking that padding reduces the object's momentum, when it only spreads the change over more time. Many students also think a stronger, stiffer container protects better, when a rigid container passes the impact through quickly. Others judge designs by appearance or effort instead of test data.
Can HS-PS2-3 be taught without sensors?
Yes. Phone slow-motion video next to a ruler gives stopping times and crush distances good enough for comparisons, and a stopwatch gives parachute fall times. Many egg-drop and parachute challenges need only paper, tape, plastic bags and string. Invented data sets like the ones on this page also let students practice evaluation.
What comes before and after HS-PS2-3?
Before it, students need Newton's second law (HS-PS2-1) and momentum (HS-PS2-2). After it, the engineering design standards such as HS-ETS1-3 ask students to evaluate solutions to complex problems with prioritized criteria and trade-offs, and energy standards such as HS-PS3-3 ask students to design devices that convert energy.
07
Related Standards
5 standards
These standards connect to HS-PS2-3: prerequisites to review first, parallel standards at the same level, and next steps that build on it.
Before this lesson
HS-PS2-1Prerequisite
Analyze data showing that net force, mass and acceleration follow Newton's second law
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