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Discover how engineers use force, time, and cushioning to protect people during crashes and impacts.
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.
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.
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.
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.
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.
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.
We can rearrange this equation to solve for force:
There is also a connection to momentum. Momentum (the amount of motion an object has) equals mass times velocity.
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.
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.
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.
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 |
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) |
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!
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.