MS-PS2-1: Using Newton's Third Law to Design for Collisions
In plain English: MS-PS2-1 is the NGSS middle school performance expectation that asks students to apply Newton's third law to design a solution to a problem in which two objects collide. In a collision the two objects push on each other with forces of equal size in opposite directions, so a good design, such as a bumper, lowers both forces. It is usually taught in grade 7 or 8 physical science.
Apply Newton's Third Law to design a solution to a problem involving the motion of two colliding objects.
Clarification Statement: Examples of practical problems could include the impact of collisions between two cars, between a car and stationary objects, and between a meteor and a space vehicle.
Assessment Boundary: Assessment is limited to vertical or horizontal interactions in one dimension.
Next Generation Science Standards · Disciplinary Core Idea: Motion and Stability: Forces and Interactions · Official standard
Students learn Newton's third law: when two objects push on each other, the two forces are always equal in size and opposite in direction. They use it to explain collisions between two cars, between a car and a wall, and between a grain of space rock and a spacecraft. Then they act as engineers. Because the two forces in a collision are always equal, a good design cannot protect one object at the expense of the other. It lowers both forces, for example with a bumper that squashes and makes the collision last longer. Students define criteria and constraints, build and test bumpers for a toy car, and choose the best design from their data.
The NGSS Clarification Statement says: "Examples of practical problems could include the impact of collisions between two cars, between a car and stationary objects, and between a meteor and a space vehicle." The Assessment Boundary says: "Assessment is limited to vertical or horizontal interactions in one dimension." Every collision on this page happens along one straight line, either sideways or up and down. All data sets are invented for teaching and labeled that way.
Learning Objectives
By the end of this lesson, students will be able to:
State Newton's third law and name the two forces in a force pair, including the object each force acts on
Use force data from a collision to show that the two forces are equal in size and opposite in direction, even when the masses are different
Explain why the object with less mass has the larger change in speed in a collision
Define criteria and constraints for a collision problem, then design, test and compare a solution that lowers the force on both objects
Use test data to choose the design that best meets the criteria and constraints, and suggest an improvement
Prior Knowledge Required
Students should already be comfortable with:
Forces as pushes and pulls that can change how an object moves 3-PS2-1
Measuring length in centimeters and mass in grams or kilograms
Using ratio language such as "twice as large" or "one third as much" 6.RP.A.1
Show a photo of a car windshield with a bug splat on it. Ask students to vote before any discussion:
Warm-Up Prompt
"A bug hits the windshield of a moving car. Which pushes harder during the hit: the bug on the car, the car on the bug, or are the pushes the same size? Which one has the bigger change in its motion?"
Record the votes. Many students say the car pushes harder because it is bigger, or because the bug is the one that gets squashed. Do not settle the question yet: the lesson returns to it with data.
Direct Instruction15 minutes
Key words. A force is a push or a pull. We measure it in newtons (N); 1 N is about the weight of a small apple. A collision is a short time when two objects push on each other. Mass is the amount of matter in an object, measured in kilograms (kg).
Newton's third law. When object A pushes on object B, object B pushes back on object A with a force of the same size in the opposite direction. The two forces are called a force pair. They act at the same moment, but on different objects, so they never cancel each other out. The same force has a bigger effect on an object with less mass: its speed changes more.
What this means for design. In a collision, a design cannot make one object feel less force than the other. It can make both forces smaller. A bumper, a crumple zone (a part of a car built to crush slowly) or a cushion makes the collision last longer and squash over a longer distance. For the same object stopping from the same speed, the average force times the stopping time stays the same, and so does the average force times the squash distance (that second rule comes from energy, which you study in MS-PS3). Double the time or the distance, and the force is about half as large. These two rules are tools for comparing designs, not new laws to memorize, and every problem that uses one states it. The law that matters most is still the third law: whatever a bumper does to the force on one object, it does to the force on the other. Use Diagram 1 with the first example.
Two cars collide
In an invented low-speed crash test, a 1,000 kg small car moving at 6 m/s hits a 2,000 kg pickup truck that is parked in neutral with its brakes off. The two lock together. Force sensors on both bumpers record an average force of 40,000 N during the crash.
Result: The car pushes on the truck with 40,000 N forward, and the truck pushes on the car with 40,000 N backward: equal and opposite. Afterward both move at 2 m/s. The car's speed changes by 4 m/s and the truck's by 2 m/s: twice the mass, half the change in speed.
A car and a stationary object
An invented test: a 1,200 kg car moving at 10 m/s hits a concrete pillar. Against bare concrete it stops in 0.05 s, with an average force of 240,000 N. Then sand-filled crash barrels are placed in front of the pillar, and the car stops in 0.50 s.
Result: The pillar pushes on the car exactly as hard as the car pushes on the pillar. The stop takes 10 times as long, so the force is one tenth as large: 240,000 ÷ 10 = 24,000 N on the car and 24,000 N on the barrels.
A meteor and a space vehicle
A grain of space rock (a meteoroid) with a mass of 0.10 g moves at 7,000 m/s relative to a space station. Engineers put a thin metal sheet, called a bumper, 10 cm in front of the station's wall.
Result: The grain and the bumper sheet push on each other with equal forces. That force is huge compared with the grain's tiny mass, so the grain breaks into a spray of small pieces. The spray spreads out across the 10 cm gap and hits the wall over a wider area, so no single spot on the wall takes the whole force.
Choosing a bumper from test data
Invented tests of three bumpers on the same 50 g toy car rolling down an 8 cm ramp into a heavy book. Criteria: average force 2.0 N or less, and the car bounces back 5 cm or less. Constraint: the bumper is at most 4 cm long. Results: sponge, squash 2.4 cm, 1.5 N, rebound 3 cm; folded paper, squash 1.2 cm, 3.0 N, rebound 1 cm; rubber band, squash 3.0 cm, 1.2 N, rebound 12 cm.
Result: Force times squash distance is 3.6 N·cm for all three. Folded paper fails the force criterion and the rubber band fails the rebound criterion, so the sponge is the best choice. Diagram 2 shows the results.
After each example, ask: "Name the force pair. Which object does each force act on? Which object changes its motion more, and why?" Then return to the warm-up: the bug and the car push on each other equally, but the bug's tiny mass means its motion changes enormously.
Guided Practice15 minutes
Pairs analyze the invented data below. In every trial, toy car A rolls at 1.2 m/s into toy car B, which is at rest. A small ball of clay on the bumpers makes the cars stick together. A force sensor on each car records the average force during the collision.
Guided practice data (invented): toy car collisions
Trial
Mass of A (kg)
Mass of B (kg)
Force on A (N)
Force on B (N)
Change in speed of A (m/s)
Change in speed of B (m/s)
1
0.20
0.20
2.4
2.4
0.6
0.6
2
0.20
0.40
3.2
3.1
0.8
0.4
3
0.20
0.60
3.6
3.6
0.9
0.3
4
0.40
0.20
3.1
3.2
0.4
0.8
Compare the two force readings in each trial. The sensors can be off by 0.1 N. Are the forces equal within that error?
In each trial, which car has the larger change in speed? Compare the ratio of the masses with the ratio of the changes in speed.
Write one claim about the forces and one claim about the changes in speed, and cite a trial as evidence for each.
Predict: car A (0.20 kg) hits a 1.00 kg car B. Which car changes speed more, and about how many times more? (Car A, about 5 times more.)
Independent Practice10-15 minutes
Each student writes a short design brief for this problem: the school gym has a 30 kg rolling rack of basketballs. When students push it into the storage room, it sometimes bumps the wall and chips the paint. Students list two criteria (what the design must do) and two constraints (limits such as cost, size, materials or time), then sketch a bumper and label the force pair during a bump.
Invented test data to use: with foam A, which squashes 1.0 cm, the average force on the wall is 1,500 N. Foam B squashes 3.0 cm in the same kind of bump. Students use the squash-distance rule to estimate the force with foam B (about 500 N, one third as much) and explain, using Newton's third law, why foam B also protects the rack.
Closure5 minutes
Exit ticket: "A 6 kg bowling ball hits a 1.5 kg pin. Is the force of the pin on the ball smaller than, the same as, or larger than the force of the ball on the pin? Why does the pin fly off while the ball hardly slows down?" (The same size; the pin has one fourth of the ball's mass, so its change in speed is about four times as large.) Revisit the warm-up votes and let students change their answers with a reason.
Differentiation Strategies
For Struggling Students
Give a force pair sentence frame: "The ___ pushes on the ___. The ___ pushes back on the ___ with the same size force in the opposite direction."
Have students act out a force pair by pressing palms with a partner while each holds a bathroom scale between their hands, and compare the two readings
Provide a design table with columns already labeled: design, squash distance, rebound, mass, meets criteria (yes or no)
For Advanced Students
Ask students to explain why a crumple zone protects people in both cars in a crash, even though it only crushes on one car
Have students test whether the squash-distance rule holds by running the same bumper from two ramp heights and comparing force times distance
Ask students to write a one-page proposal comparing two meteoroid shield designs, using one criterion and two constraints (mass and thickness)
Assessment Guidance
What to Look For
Check that students name both forces in a pair and say which object each acts on, not "the car's force and the wall's force" without objects. Look for students who still say the bigger or faster object pushes harder, and for students who think equal forces mean equal damage or equal changes in speed. In design work, check that the chosen solution is backed by test data and that students separate criteria (what the design must do) from constraints (limits on the design).
02
Classroom Activities
3 Activities
1
Spring Scale Pull: Finding the Force Pair
15 minPairs
Partners hook two 10 N spring scales together and pull. A spring scale is a tool with a spring inside that shows the size of a pull in newtons. Pairs test whether one scale can ever read more than the other, even when only one partner pulls.
Procedure
Hook the two scales together. Each partner holds one scale level, at waist height.
Trial 1: both partners pull gently. Trial 2: both pull a bit harder. Trial 3: partner A holds still while partner B pulls. Trial 4: hook one scale to a sturdy door handle and pull the other.
Read both scales at the same moment in each trial, using phone video if the readings move.
Safety: never pull past 10 N, and never let go of a stretched scale; release slowly.
Sample Data (invented)
Trial 1: 2.0 N and 2.0 N. Trial 2: 3.5 N and 3.5 N. Trial 3: 4.0 N and 4.0 N. Trial 4: 5.0 N on the scale students pull and 5.0 N on the scale hooked to the door handle.
Discussion Questions
In every trial the two readings match. Does it matter who pulls and who holds still?
In Trial 4, what pulled on the second scale? What does that tell you about a wall or a pillar in a crash?
How is pulling on the scales like two cars pushing on each other during a collision?
Challenge Variation
Tape a small ceramic magnet to the back of each of two toy cars, with like poles facing. Push the cars together, then let go: they push each other apart. Add clay to one car and compare how far each car rolls.
2
Toy Car Bumper Design Challenge
35 minGroups of 3
Groups design, build and test a bumper for a 50 g toy car that rolls down an 8 cm ramp into a heavy book standing against a wall (the barrier). They record the squash distance from slow-motion video, estimate the force, and choose the best design with a decision table.
The Design Brief
Criteria (what the bumper must do): squash at least 2.0 cm, so the average force on the car and the book stays at about 1.8 N or less, and let the car bounce back 5 cm or less.
Constraints (limits): the bumper is at most 4 cm long and at most 5 g (weigh it on a kitchen scale), uses only the materials tray (sponge, cotton balls, straws, index cards, rubber bands, tape), and must be built in 15 minutes.
For this car and ramp, earlier tests gave about 3.6 N·cm for average force times squash distance, so the force in newtons is about 3.6 ÷ (squash distance in cm).
Procedure
Build the ramp: one end of a board on books, 8 cm high. Tape a ruler on the floor next to the book.
Release the car from the same mark each time. Film the hit in slow motion and read the squash distance and the rebound against the ruler. Run three trials.
Record the mean squash distance and the mean rebound, then improve the design once and test again.
Sample Data (invented)
Cotton ball pack: squash 2.0 cm, rebound 2 cm, 3 g. Straw accordion: squash 1.5 cm, rebound 1 cm, 4 g. Rubber band sling: squash 3.2 cm, rebound 9 cm, 2 g. Sponge and card: squash 2.6 cm, rebound 2 cm, 6 g.
Discussion Questions
Only one sample design meets every criterion and constraint. Which one?
The sponge and card design gives a lower force than the cotton ball pack. Why is it still rejected?
Using Newton's third law, explain why a good bumper protects the book as well as the car.
Modification for Distance Learning
Students build a bumper at home for any small toy car, roll it down a ramp made from a cereal box, and share their video and data table on the class board. Others score the design against the brief.
3
Collision Case Files
20 minGroups of 3-4
Groups rotate through four case cards, each describing a real kind of collision with invented numbers. For each case they draw the force pair, decide which object changes its motion more, and suggest one design improvement with one criterion and one constraint.
Case Cards
Case 1, railroad cars: a 30,000 kg boxcar rolling at 1.0 m/s couples with a 30,000 kg boxcar at rest. Spring buffers sit on the ends of both cars.
Case 2, boat and dock: a 2,000 kg boat drifts into a dock at 0.5 m/s. The dock has old tires hung on its side, and it is built on posts sunk into the lake bed.
Case 3, satellite and space dust: a 1 g fleck of paint from an old rocket hits a 500 kg satellite at 10,000 m/s.
Case 4, model lander (vertical): a 0.8 kg model space lander drops the last few centimeters onto a landing pad. Its legs have small springs.
Discussion Questions
In which two cases does one object barely change its motion at all? Explain why, using mass.
In Case 1, the two boxcars have equal masses. How do their changes in speed compare?
In Case 3, the satellite has 500,000 times the mass of the paint fleck. Does it feel a smaller force than the fleck? What does it feel instead?
Modification for Distance Learning
Post each case as a slide. Students draw the force pair on the slide with an annotation tool and type their design improvement in the speaker notes.
03
Diagrams & Visual Aids
2 diagrams
Diagram 1: The Force Pair in a Car and Pickup Truck Collision
Invented crash test from the first worked example. The two force arrows are the same length because the forces are equal in size; they point in opposite directions and act on different vehicles. The bars, drawn to scale, show that the car (half the mass of the truck) has twice the change in speed.
Diagram 2: Bumper Test Results Against the Force Criterion
Invented test data for three bumpers on a 50 g toy car, drawn to scale. The dashed line marks the criterion of 2.0 N. The sponge (light bar) is the only bumper below the line that also keeps the rebound at 5 cm or less.
04
Homework Assignment
~30 min
MS-PS2-1 Homework: Force Pairs and Collision Designs
Directions: All data are invented for practice, and every collision happens along one straight line. For each problem, name the force pair, show your calculations with units, and explain your answer in one or two sentences.
Part 1: Force Pairs in Collisions (Problems 1-2)
At a lake, two bumper boats collide head-on. Boat A with its rider has a mass of 200 kg; boat B with its rider has a mass of 100 kg. A sensor on boat A reads an average force of 600 N during the bump. (a) What force does boat A exert on boat B, and in which direction? (b) Which boat has the larger change in speed, and how many times larger? (c) The rider in boat B says, "The big boat hit us harder." Use Newton's third law to respond.
A 25 kg delivery robot rolls into a hallway wall at 1.2 m/s and stops in 0.10 s. A sensor shows an average force of 300 N on the wall. (a) What force does the wall exert on the robot, and in which direction? (b) The wall does not seem to move at all. Explain why, using mass. (c) Engineers add a foam bumper so the robot stops in 0.30 s instead. Use the stopping-time rule to find the new average force on the robot and on the wall.
Part 2: Analyzing Design Test Data (Problems 3-4)
A library needs a bumper for its 40 kg rolling book cabinet. Each material was tested three times in the same bump. Foam pipe insulation (3 cm thick, $6): 390, 410, 400 N. Rubber strip (2 cm thick, $4): 520, 540, 530 N. Pool noodle half (6 cm thick, $3): 300, 320, 310 N. Carpet scrap (1.5 cm thick, $2): 650, 670, 660 N. Criterion: mean force 450 N or less. Constraints: no thicker than 5 cm (to fit through doorways) and no more than $8. (a) Find the mean force for each material. (b) Which materials meet the criterion? (c) Which one material meets the criterion and both constraints?
Engineers test shields for a spacecraft by firing tiny pellets at a thin bumper sheet placed in front of an 8 mm thick wall. They change only the gap between the sheet and the wall. Invented results for the dent depth in the wall: gap 0 cm, 6.0 mm; gap 4 cm, 3.0 mm; gap 8 cm, 1.6 mm; gap 12 cm, 1.0 mm; gap 16 cm, 0.8 mm. (a) Describe the pattern. (b) Is the dent depth proportional to the gap? Explain with two pairs of values. (c) The criterion is a dent of 2.0 mm or less, and the constraint is that the shield may add at most 10 cm to the spacecraft. Which tested gap should engineers choose? (d) Use Newton's third law to explain what happens to a pellet when it hits the bumper sheet.
Part 3: Design and Argue (Problems 5-6)
A 1.5 kg drone lands straight down on a pad. With stiff plastic legs, the legs squash 1.0 cm and the average force during the landing is 90 N. For the same landing, the average force times the squash distance stays the same. (a) What force does the pad exert on the drone, and what force does the drone exert on the pad, with stiff legs? Give both directions. (b) Find the average force for spring legs that squash 3.0 cm and for foam pads that squash 2.0 cm. (c) The criterion is a landing force of 50 N or less. The constraint is that new legs may add at most 40 g. Spring legs add 60 g and foam pads add 25 g. Which design should the engineers choose?
A classmate claims: "In a crash between a big van and a small car, the van feels a bigger force because it hits harder." In an invented crash test, force sensors on a 1,800 kg van and a 1,200 kg car both read an average of 36,000 N. The car's speed changed by 3.0 m/s and the van's by 2.0 m/s. (a) Write a claim, evidence and reasoning response to the classmate. (b) Explain why the car's speed changed more. (c) Suggest one design feature that would lower the force on the people in both vehicles, and explain why it works.
Rubric
Criterion
Full Credit (2 pts)
Partial Credit (1 pt)
Force Pairs
Both forces named with the object each acts on, equal size and opposite directions stated
Forces called equal, but objects or directions missing
Using Data
Means, ratios and rules applied correctly with units; changes in speed linked to mass
Correct method with one calculation or unit error
Design Choice
Choice checked against every criterion and constraint, with data cited
Choice stated, but one criterion or constraint not checked
05
Quiz: 20 Questions
Interactive, with answers
Instructions
All data are invented, and every collision happens along one straight line. 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
When a hammer hits a nail, how does the force of the nail on the hammer compare with the force of the hammer on the nail?
Answer: D
Newton's third law says the two forces in a pair are equal in size and opposite in direction, whatever the masses. Choice B mixes up force with change in motion: the nail's small mass means its motion changes more, not that it pushes less. Choice C is wrong because the nail pushes back even while it moves.
Question 2 of 20 · Multiple Choice
A 0.30 kg toy car collides with a 0.90 kg toy truck. A sensor on the car reads an average force of 3.6 N. What average force acts on the truck?
Answer: A
The force on the truck is the other half of the force pair, so it is 3.6 N in the opposite direction. Choice B divides by 3 because the truck has 3 times the mass, and choice C multiplies by 3; mass changes the effect of the force, not its size. Choice D forgets that the truck pushes back even if it starts at rest.
Question 3 of 20 · Multiple Choice
In the same collision, the 0.30 kg car's speed changes by 0.75 m/s. By how much does the 0.90 kg truck's speed change?
Answer: C
The forces are equal, and the truck has 3 times the mass, so its change in speed is one third as large: 0.75 ÷ 3 = 0.25 m/s. Choice A multiplies by 3 instead of dividing. Choice B assumes equal forces give equal changes in speed. Choice D subtracts the 0.60 kg difference in mass from 0.75 instead of dividing by the mass ratio.
Question 4 of 20 · Multiple Choice
Why don't the two forces in a force pair cancel each other out?
Answer: B
Forces can only cancel when they act on the same object. In a force pair, one force acts on each object, so each object feels its own unbalanced push. Choice A contradicts Newton's third law, and choice C is wrong because the two forces start and stop at the same moment.
Question 5 of 20 · Multiple Choice
A car hits a pillar and stops in 0.08 s with an average force of 150,000 N. With crash barrels in front of the pillar, the same car at the same speed stops in 0.40 s. For the same car stopping from the same speed, average force × stopping time stays the same. What is the new average force on the car?
Answer: D
Force times stopping time stays the same: 150,000 × 0.08 = 12,000, and 12,000 ÷ 0.40 = 30,000 N. The stop lasts 5 times as long, so the force is one fifth. Choice A multiplies by 5 instead of dividing. Choice B ignores the barrels. Choice C divides by 4, using the extra time 0.40 - 0.08 = 0.32 s instead of the whole stopping time.
Question 6 of 20 · Multiple Choice
In the crash with the barrels, how does the force of the barrels on the car compare with the force of the car on the barrels?
Answer: A
The two forces are a force pair, so they are equal in size and opposite in direction at every moment of the crash. Choice C uses the idea that a moving object pushes harder, which the force sensor data on this page do not support. Choice D is wrong: crushing barrels still push back on the car, only with a smaller force.
Question 7 of 20 · Multiple Choice
A 1 g grain of space dust hits a 400 kg satellite. Which statement is correct?
Answer: C
Newton's third law makes the forces equal. The grain has far less mass (1 g compared with 400,000 g), so the same force changes its motion far more. Choice A confuses equal forces with equal changes in motion. Choices B and D each claim one object pushes harder, which breaks the third law.
Question 8 of 20 · Multiple Choice
A group writes a design brief for a toy car bumper. Which item is a constraint (a limit on the design), not a criterion (something the design must do)?
Answer: B
A cost limit restricts how the bumper can be made, so it is a constraint. Choices A, C and D each describe what a successful bumper must achieve, so they are criteria.
Question 9 of 20 · Multiple Choice
Four bumpers were tested on a toy car. Criterion: average force 1.8 N or less. Constraint: bumper mass 6 g or less. Felt pad: 1.6 N, 7 g. Bubble wrap: 1.9 N, 4 g. Cardboard spring: 2.2 N, 3 g. Foam cube: 1.7 N, 5 g. Which bumper meets both?
Answer: D
The foam cube gives 1.7 N (at most 1.8 N) and weighs 5 g (at most 6 g). The felt pad has the lowest force but breaks the mass constraint, which is the trap in choice A. Bubble wrap and the cardboard spring are light enough, but their forces are above 1.8 N.
Question 10 of 20 · Multiple Choice
A 0.60 kg basketball hits the gym floor. During the bounce, the floor pushes up on the ball with an average force of 90 N. What force does the ball exert on the floor?
Answer: C
The ball and the floor form a force pair along a vertical line, so the ball pushes down on the floor with 90 N. Choice A has the right size but the wrong direction. Choice B is the ball's weight (0.60 × 9.8 ≈ 5.9 N), a different force. Choice D assumes that a force on the floor would have to make it move: the ball does push on the floor with 90 N, but the floor is attached to Earth, whose huge mass is why its motion does not visibly change.
Question 11 of 20 · Multiple Choice
A student wants to test the claim "In a crash, the heavier toy car feels the larger force." Which measurement would test the claim directly?
Answer: A
The claim is about forces, so the direct test measures the force on each car at the same time. Choice C measures changes in motion, which depend on mass as well as force, so it cannot separate the two. Choices B and D measure speed or mass only.
Question 12 of 20 · Multiple Choice
In tests of one toy truck, the average force times the squash distance stays about the same. With a bumper that squashes 2.0 cm, the barrier pushes on the truck with an average force of 3.0 N. With a bumper that squashes 6.0 cm in the same test, what average force does the truck exert on the barrier?
Answer: B
Force times distance is 3.0 × 2.0 = 6.0 N·cm, so the force on the truck is 6.0 ÷ 6.0 = 1.0 N: three times the squash, one third of the force. By Newton's third law, the truck pushes on the barrier with the same 1.0 N in the opposite direction. Choice A multiplies 3.0 by 3 instead of dividing. Choice C ignores the longer squash. Choice D divides 3.0 by 6.0 without first multiplying by 2.0.
Question 13 of 20 · Multiple Choice
Two cars are about to collide. Which design change lowers the force on both cars?
Answer: D
A crumple zone makes the crash last longer, and because the forces on the two cars are a pair, a smaller force on one car means a smaller force on the other. Choice A shortens the crash and raises both forces. Choice B does not lower the force pair either: the lighter car would take an even larger change in speed.
Question 14 of 20 · Multiple Choice
During a crash, a truck pushes on a car. What is the other force in this force pair?
Answer: B
The partner of "truck pushes on car" swaps the two objects: "car pushes on truck." Choices A, C and D are real forces, but each involves a different pair of objects (road and car, Earth and truck, truck and road), so none is the partner.
Question 15 of 20 · Short Answer
A 0.25 kg toy car and a 0.75 kg toy car collide head-on. A sensor on the 0.75 kg car reads an average force of 2.4 N. (a) What force acts on the 0.25 kg car? (b) Which car has the larger change in speed, and how many times larger?
(a) 2.4 N, in the opposite direction to the force on the 0.75 kg car (Newton's third law). (b) The 0.25 kg car. It has one third of the mass, so the same force changes its speed 3 times as much.
Question 16 of 20 · Short Answer
A school wants a bumper for its 15 kg hallway robot, which sometimes bumps into doors. Write two criteria and two constraints for the design, and use Newton's third law to explain why a good bumper protects the door as well as the robot.
Sample criteria: the average force in a bump stays below a set value (for example 100 N); the robot does not bounce back more than a few centimeters. Sample constraints: the bumper adds no more than 1 kg; it costs no more than $10; it fits within the robot's width. Reasoning: the robot and the door push on each other with equal and opposite forces, so a bumper that lowers the force on the robot by making the bump last longer lowers the force on the door by the same amount.
Question 17 of 20 · Short Answer
A group tests its bumper three times and records average forces of 1.9 N, 2.1 N and 2.3 N. The criterion is a mean force of 2.0 N or less. Does the bumper meet the criterion? Suggest one change.
The mean is (1.9 + 2.1 + 2.3) ÷ 3 = 2.1 N, which is above 2.0 N, so the bumper does not meet the criterion. One change: make the bumper softer or longer (within the size constraint) so it squashes farther, which makes the collision last longer and lowers the force.
Question 18 of 20 · Short Answer
A high jumper lands on her back on a foam landing pit. During the landing, the pit pushes up on her with an average force of 1,800 N. How hard does she push on the pit, and in which direction? Why does a thicker, softer pit lower this force?
She pushes down on the pit with 1,800 N: the two forces are a vertical force pair. A thicker, softer pit squashes farther and makes the landing last longer, so both forces in the pair become smaller.
Question 19 of 20 · Short Answer
A grain of space dust hits a space station window and leaves a tiny chip, and the grain itself is destroyed. A student says, "The window must have pushed harder on the grain than the grain pushed on the window." Use Newton's third law to respond.
The student is wrong: the window and the grain push on each other with forces of equal size in opposite directions. The results differ because the objects differ. The grain has a tiny mass, so the force changes its motion enormously and breaks it apart; the window is far more massive and is held by the station, so the same force only chips its surface.
Question 20 of 20 · Short Answer
For one toy truck, average force times squash distance is 4.8 N·cm. A classmate designs a very stiff bumper that squashes only 0.4 cm and says, "It doesn't squash, so it protects the truck best." A sponge bumper squashes 1.6 cm. Find the force with each bumper and respond to the classmate.
Stiff bumper: 4.8 ÷ 0.4 = 12 N. Sponge bumper: 4.8 ÷ 1.6 = 3 N. The classmate has it backward: a bumper that squashes farther makes the collision last longer, so the force on the truck, and (by Newton's third law) on whatever it hits, is 4 times smaller with the sponge.
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Frequently Asked Questions
10 Questions
What does MS-PS2-1 mean?
MS-PS2-1 asks middle school students to use Newton's third law to design a solution to a problem in which two objects collide. Students must show that the two forces in a collision are equal and opposite, then design and test something, such as a bumper or cushion, that lowers the force on both objects. The code means Middle School, Physical Science, core idea 2 (Motion and Stability: Forces and Interactions), performance expectation 1.
What grade is MS-PS2-1 taught in?
It is a grades 6-8 standard, and schools place it in different years. Many teach it in grade 7 or grade 8 physical science, often in the same unit as MS-PS2-2 on forces and motion. The math needed is ratios and the mean of a few numbers.
What is the assessment boundary for MS-PS2-1?
The official Assessment Boundary limits assessment to vertical or horizontal interactions in one dimension. Collisions along a straight line, such as two cars meeting head-on or a ball bouncing straight up from the floor, are in scope. Glancing collisions at an angle, where objects fly off in different directions, are beyond this standard.
If the forces are equal, why does the smaller object get hurt more?
Because the same force has a bigger effect on an object with less mass. In a crash between a small car and a big truck, both feel the same force, but the small car's speed changes more, so the people inside it are jolted more.
Does Newton's third law work when one object is not moving?
Yes. A wall, a pillar or a parked car pushes back on whatever hits it with the same size force, even though it may not move. A wall attached to a building is joined to Earth, and its enormous mass is why its motion does not visibly change.
What are criteria and constraints in an engineering design?
Criteria are what a design must do to count as a success, such as keeping a force below a set value. Constraints are limits on the design, such as cost, size, mass, materials or build time. A good design meets every criterion while staying within every constraint, and students check both with test data.
Do students need force sensors for MS-PS2-1?
No. Pairs of spring scales show equal and opposite forces with no electronics, and slow-motion phone video lets students measure how far a bumper squashes. If a school has force sensors, two of them on colliding toy cars make the equal readings easy to see.
What is a good design project for Newton's third law?
A toy car bumper challenge works well: students design a bumper that lowers the force when a car rolls into a barrier, within limits on size, mass and materials. Other options are cushions for a dropped package, landing legs for a model lander, and a shield that protects a model spacecraft from fast-moving particles.
Is MS-PS2-1 about momentum?
Not directly. MS-PS2-1 focuses on the force pair and on designing for collisions. Students notice that the lighter object's speed changes more, and in high school, HS-PS2-2 turns that pattern into the conservation of momentum.
How does MS-PS2-1 connect to high school physics?
It prepares students for HS-PS2-2, where they use math to show that momentum is conserved in collisions, and HS-PS2-3, where they design and refine a device that reduces the force in a collision using the idea of stopping time. The engineering steps also connect to MS-ETS1-2, evaluating competing designs.
07
Related Standards
5 standards
These standards connect to MS-PS2-1: prerequisites to review first, parallel standards at the same level, and next steps that build on it.
Before this lesson
3-PS2-1Prerequisite
Investigate how balanced and unbalanced forces affect an object's motion
Lesson coming soon
Alongside
MS-PS2-2Parallel
Plan an investigation showing that change in motion depends on net force and mass
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