Design a solution that reduces or changes the effects of a collision using force interactions - Middle School Physical Science

SECTION 1

Why Do We Need Collision Protection?

Have you ever wondered why cars have bumpers? Or why football players wear helmets? People have been designing ways to reduce the damage from collisions (events where two objects crash into each other) for hundreds of years. Early solutions were simple, like straw padding in horse carriages. Today, engineers use science to build amazing safety devices.

This is our anchoring phenomenon: When an egg is dropped from a height, it breaks on a hard floor—but it can survive if wrapped in bubble wrap. Why does the same egg, falling the same distance, have such different outcomes? The answer lies in how forces interact during a collision.

1885 • First Car Bumper Concepts

Early automobiles had no protection. Engineers began adding rubber strips to the front and back of vehicles to soften impacts.

1952 • First Crash Test Dummies

Engineers created human-shaped models to test how forces affect the body during car crashes. This led to better seat belt and airbag designs.

1968 • Seat Belt Laws Begin

The United States required all new cars to have seat belts. Seat belts spread the force of a collision across a larger area of your body.

1998 • Airbags Required in All Cars

Airbags became standard in all new U.S. cars. They inflate in milliseconds and increase the time over which a collision force acts on a person.

2020s • Modern Crumple Zones and Smart Materials

Today's cars are designed so the front and rear sections crumple during a crash. This absorbs energy and reduces the force that reaches passengers.

Throughout history, one big question has driven these inventions: How can we change the way forces act during a collision so people stay safe? In this lesson, you will learn the science behind that question. Then you will think like an engineer and design your own collision solution.

SECTION 2

Core Principles of Collisions and Forces

To design a collision solution, you need to understand a few key ideas. A force is a push or pull on an object. During a collision, forces can be very large and act over a very short time. The bigger the force, the more damage it can cause. Engineers use science principles to make those forces smaller or spread them out.

Newton's Third Law

When two objects collide, they push on each other with equal and opposite forces. If a ball hits a wall, the wall pushes back on the ball with the same amount of force.

Impulse and Force Over Time

Impulse is the total effect of a force acting over time. The same impulse can come from a big force over a short time, or a small force over a longer time. Making the collision last longer reduces the peak force.

Energy Transfer in Collisions

When objects collide, kinetic energy (energy of motion) is transferred or changed into other forms like heat or sound. Cushioning materials absorb some of this energy so less is transferred to you.

Force and Area

Spreading a force over a larger area reduces the pressure at any single point. A seat belt spreads force across your chest instead of concentrating it at one spot.

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KEY TAKEAWAY

Think of catching a water balloon. If you catch it with stiff hands, it pops—the force is big and sudden. But if you let your hands move backward as you catch it, the force is spread over more time, and the balloon survives. Collision safety works the same way: increase the time, increase the area, or absorb energy to reduce the damaging force.

SECTION 3

Visualizing How Cushioning Changes Force

The diagram below shows what happens when the same ball hits two different surfaces. On the left, it hits a hard wall and stops almost instantly. On the right, it hits a foam pad and takes much longer to stop. Both collisions change the ball's speed by the same amount. But the peak force is very different. Look at how the force-vs-time graphs compare.

Both graphs show a ball stopping from the same speed. The left graph shows a tall, narrow spike (high force, short time). The right graph shows a low, wide curve (low force, long time). The area under each curve—the impulse—is roughly the same.

Notice how the areas under both curves are about the same size. That area represents the impulse, the total push needed to stop the ball. The cushion does not change the impulse. It changes how that impulse is delivered. A longer time means a smaller peak force. This is the Crosscutting Concept of Cause and Effect in action: changing the time of collision causes the force to change.

SECTION 4

The Math Behind Safer Collisions

You do not need advanced math to understand collision safety. There is one simple relationship that explains almost everything. It connects force, impulse, and time.

IMPULSE EQUATION

Impulse = Force × Time

Impulse is measured in newton-seconds (N·s). Force is in newtons (N). Time is in seconds (s). If the impulse stays the same but time increases, the force must decrease.

We can rearrange this equation to solve for force:

FORCE FROM IMPULSE

Force = Impulse ÷ Time

This shows that if the time of collision doubles, the force is cut in half. That is exactly what cushions and crumple zones do.

There is also a connection to momentum. Momentum (the amount of motion an object has) equals mass times velocity.

MOMENTUM

Momentum = Mass × Velocity

Mass is in kilograms (kg). Velocity is in meters per second (m/s). Impulse equals the change in momentum. So: Force × Time = Mass × Change in Velocity.

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Science Practice Spotlight

When you use the impulse equation to predict how a design will reduce force, you are using the Science and Engineering Practice of Using Mathematics and Computational Thinking. Engineers do this every day when designing safety gear!

SECTION 5

Three Strategies Engineers Use to Reduce Collision Effects

Engineers do not just guess when they design safety features. They use three main strategies based on the science of forces. Each strategy targets a different part of the collision. Let's explore them with a diagram.

Engineers combine these three strategies in real-world designs. A car uses all three: the crumple zone increases time, the seat belt spreads force, and the airbag absorbs energy.

Most real-world safety devices use more than one strategy at the same time. A bicycle helmet, for example, spreads force across your head (Strategy 2) and the foam liner crushes to absorb energy (Strategy 3). When you design a collision solution, think about how you can combine strategies for maximum protection.

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Crosscutting Concept: Systems and System Models

A car's safety system includes many parts working together: crumple zones, airbags, seat belts, and headrests. Scientists model the car-and-passenger as a system and study how forces flow through each part to improve the overall design.

SECTION 6

Worked Example: Designing an Egg Drop Container

Your teacher gives you this challenge: Drop a raw egg from 2 meters onto a hard floor without breaking it. You have cardboard, bubble wrap, rubber bands, and tape. Let's walk through the engineering design process step by step.

Egg Drop Engineering Design Challenge

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Step 1 — Define the Problem

The egg breaks because the collision force is too large. When the egg hits the floor, it goes from moving to stopped in a very short time. We need to reduce the force that reaches the egg.

Goal: Reduce the peak collision force below the level that cracks the eggshell.
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Step 2 — Identify the Science

Using the impulse equation: Force = Impulse ÷ Time. The impulse is set by the egg's mass and speed at impact. The egg has a mass of about 0.06 kg. Falling 2 meters, it reaches about 6.3 m/s. So the impulse is: 0.06 kg × 6.3 m/s = 0.378 N·s.

Impulse ≈ 0.378 N·s (this cannot change—we must change the time).
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Step 3 — Choose Design Strategies

We will use all three strategies. Bubble wrap will increase the collision time and absorb energy (Strategies 1 and 3). A cardboard frame will spread the force over the egg's whole surface (Strategy 2). Rubber bands will suspend the egg inside, adding more cushioning.

Design: Egg suspended by rubber bands inside a box lined with bubble wrap.
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Step 4 — Calculate the Expected Force

Without protection, the egg stops in about 0.002 seconds. Force = 0.378 ÷ 0.002 = 189 N. That is way too much. With bubble wrap, the collision time increases to about 0.05 seconds. Now Force = 0.378 ÷ 0.05 = 7.56 N. That is about 25 times less force!

Predicted force with cushioning ≈ 7.56 N (safe for the egg!)
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Step 5 — Test and Improve

After the first test, check if the egg survived. If it cracked, ask: Did the cushioning compress all the way? If so, add more layers. Did the egg hit the frame directly? If so, adjust the rubber band suspension. This is the iterative design process—test, evaluate, and improve.

Revise the design based on evidence from each test.

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ENGINEERING PRACTICE

This worked example uses the Science and Engineering Practice of Designing Solutions. Real engineers follow the same cycle: define the problem, apply science, build, test, and improve. You can do this too!

SECTION 7

Comparing Collision Safety Designs

Different safety devices use different combinations of the three strategies. The table below compares several common designs. Notice how each one has strengths and limitations.

Safety Device	Main Strategy	Strengths	Limitations
Airbag	Increases time; absorbs energy	Deploys in milliseconds; greatly reduces head and chest injuries	Only works once; can injure small passengers if too close
Seat Belt	Spreads force over area; increases time	Reusable; keeps passengers in place; works in all crash types	Can cause bruising in severe crashes; only works if worn correctly
Bike Helmet	Spreads force; absorbs energy	Lightweight; protects the brain from direct impact	Must be replaced after one crash; does not protect the face
Crumple Zone	Increases time; absorbs energy	Built into the car; absorbs huge amounts of energy	Car is destroyed and cannot be reused; expensive to repair
Bubble Wrap	Increases time; absorbs energy	Cheap; easy to use; protects fragile objects during shipping	Not strong enough for high-speed impacts; single-use

Comparison of common collision safety devices

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DESIGN TRADE-OFFS

No single design is perfect for every situation. Think about choosing a backpack for school. A heavy-duty backpack protects your laptop great, but it is bulky. A light bag is easy to carry but offers less padding. Engineers always balance protection, cost, weight, and reusability when designing collision solutions. This is called a trade-off.

SECTION 8

From Egg Drops to Real-World Engineering

The ideas you are learning now are the same ones professional engineers use. In high school and college, you will go deeper into the math behind collisions. Here is a preview of how these concepts grow.

What You Learn Now	What Comes Next
Force = Impulse ÷ Time (simple version)	Calculus-based impulse: integrating force over time
Momentum = Mass × Velocity	Conservation of momentum in two- and three-dimensional collisions
Cushioning absorbs energy	Stress-strain analysis of materials, elastic vs. plastic deformation
Designing an egg drop container	Computer simulations of full car crash tests (finite element analysis)

How middle school collision concepts connect to advanced engineering

NASA engineers design landing systems for spacecraft using these exact principles. When the Mars rover Curiosity landed in 2012, it used a sky crane and airbag system. The airbags increased the landing time and absorbed energy—just like your egg drop cushioning!

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Crosscutting Concept: Stability and Change

Collision safety is all about stability and change. A well-designed safety system keeps the passenger stable (not moving suddenly) even as the vehicle changes speed rapidly. The system absorbs the change so the person does not have to.

SECTION 9

Practice Problems

Question 1: CONCEPTUAL

A gymnast lands on a thick foam mat instead of a concrete floor. Which best explains why the mat reduces injury? A) The mat removes all the force from the landing. B) The mat increases the time of the collision, so the force is smaller. C) The mat makes the gymnast lighter. D) The mat increases the gymnast's speed.

Show answer

B is correct. The foam mat compresses during the landing, increasing the time it takes for the gymnast to stop. Since impulse = force × time, if the time gets bigger, the force gets smaller. Option A is wrong because the mat does not remove force—it reduces it. The gymnast still experiences a force. Option C is wrong because the mat does not change the gymnast's mass. Option D is wrong because the mat actually slows the gymnast down, not speeds them up.

Question 2: BASIC CALCULATION

A 0.5 kg ball is moving at 4 m/s and hits a wall, stopping completely. The collision takes 0.01 seconds. What is the average force on the ball? A) 2 N B) 20 N C) 200 N D) 2,000 N

Show answer

C is correct. First, find the impulse: Impulse = Mass × Change in Velocity = 0.5 kg × 4 m/s = 2 N·s. Then, Force = Impulse ÷ Time = 2 N·s ÷ 0.01 s = 200 N. Option A (2 N) is just the impulse value, not divided by time. Option B (20 N) would result from dividing by 0.1 s, not 0.01 s. Option D (2,000 N) would result from dividing by 0.001 s, which is too short.

Question 3: INTERMEDIATE

Using the same ball from Problem 2 (0.5 kg, 4 m/s), you add a foam pad to the wall. Now the collision takes 0.05 seconds. By how much does the average force decrease? A) The force drops from 200 N to 40 N — a decrease of 160 N. B) The force drops from 200 N to 100 N — a decrease of 100 N. C) The force drops from 200 N to 10 N — a decrease of 190 N. D) The force stays the same because the ball has the same mass.

Show answer

A is correct. The impulse is still 2 N·s (it does not change). With the foam pad, Force = 2 N·s ÷ 0.05 s = 40 N. The force decreased from 200 N to 40 N, which is a drop of 160 N. Option B would result from doubling the time, not making it 5 times longer. Option C would result from a collision time of 0.2 s. Option D is wrong because force depends on time, not just mass. Increasing the collision time does change the force.

Question 4: APPLIED

Your team is designing a phone case to protect a phone dropped from a table (about 0.8 m high). You test two designs. Design A uses a hard plastic shell. Design B uses a soft rubber bumper. In testing, Design A stops the phone in 0.005 s and Design B stops it in 0.03 s. Which design better reduces the collision force, and which collision safety strategy does it mainly use? A) Design A, because hard plastic is stronger. B) Design B, because it increases the collision time. C) Design A, because it spreads the force over a larger area. D) Both designs are equally effective.

Show answer

B is correct. Design B has a collision time 6 times longer than Design A (0.03 s vs. 0.005 s). Since Force = Impulse ÷ Time, the longer time means a smaller force on the phone. This is Strategy 1: Increase Collision Time. Option A is wrong because being 'stronger' does not mean it reduces force—it actually transmits force faster. Option C is wrong because a hard shell does not necessarily spread force over more area than a rubber bumper. Option D is wrong because the different collision times produce very different forces.

Question 5: CRITICAL THINKING

A car company designs two different bumper systems. System X has a very stiff bumper that barely dents in a crash. System Y has a bumper that crumples significantly. Both cars have the same mass and are tested at the same speed. A student says, "System X is better because it doesn't get damaged." Use what you know about force, impulse, and energy to evaluate the student's claim. A) The student is correct. Less damage to the car means less force on the passengers. B) The student is incorrect. System Y crumples, which increases the collision time, reduces the peak force, and absorbs more kinetic energy — protecting the passengers better. C) The student is correct. System X bounces back, which means it absorbed more energy. D) Both systems are equally safe because they have the same mass and speed.

Show answer

B is correct. This is a key insight: the purpose of a crumple zone is to get damaged so the passengers do not. When System Y crumples, it increases the time of collision significantly. Using Force = Impulse ÷ Time, a longer time means a smaller force reaches the passenger cabin. The crumpling also converts kinetic energy into heat and deformation energy (Energy and Matter crosscutting concept). Option A is wrong because less damage to the bumper means the collision was stiffer and shorter—producing a larger force. Option C is wrong because bouncing back actually means the bumper did not absorb the energy; it returned it. Option D is wrong because even though mass and speed are the same (same impulse), the collision time is different, so the forces are different.

SUMMARY

Lesson Summary

Summary

In this lesson, you explored how engineers design solutions to reduce or change the effects of collisions using force interactions. You learned that during a collision, objects experience forces described by Newton's Third Law (equal and opposite). The key equation Force = Impulse ÷ Time shows that increasing the collision time reduces the peak force. Engineers use three main strategies: increasing time (cushions, crumple zones), spreading force over a larger area (seat belts, helmets), and absorbing or redirecting energy (foam, crumple zones).

You practiced the Science and Engineering Practice of Designing Solutions through a worked egg drop example. You connected Cause and Effect (changing collision time causes force to change), Energy and Matter (kinetic energy is transferred or absorbed during collisions), and Systems and System Models (viewing a car's safety features as an integrated system). Remember: the best designs use multiple strategies and are improved through iterative testing.