In this blog post, we’ll explore the principles behind how soft cushioning materials reduce impact to protect objects and people, and how those principles are applied in everyday life.
The Principle Behind Soft Cushioning Materials That Absorb Impact
Although it’s sometimes mentioned today as one of the gifts women would rather not receive, just over a decade ago, one of the most representative gifts for expressing devotion and sincerity to a loved one was a bottle filled with a thousand origami cranes. But what would have happened if the box containing those carefully crafted origami cranes had been nothing but hard? Even the slightest bump would have caused the beautifully wrapped bottle to shatter into pieces inside the box, and the recipient might have felt danger rather than being moved by the gesture. The reason the paper cranes can safely deliver the bottle is that they apply mechanical engineering principles using cushioning materials. So, what principles does mechanical engineering use to safely protect objects?
Objects break due to the impact force generated when they are struck. Strictly speaking, damage occurs when the impact force acting on an object exceeds the stress the object can withstand. Here, “stress” refers to the resistance that arises within an object when an external force is applied. As the impact force increases, so does the stress applied to the object; therefore, once the impact force exceeds a certain level, the object is damaged.
Impact momentum and impact force are distinct concepts. Impact momentum refers to the total effect of forces that change an object’s state of motion, while impact force refers to the force acting between two objects at the moment of collision. This is easier to understand when analogized to the situation of lifting a heavy load. A light load can be held for a long time, but a heavy load is tiring even when held for a short time. Assuming the same level of physical strain, a light load can be held for a long time, while a heavy load can only be held for a short time. Similarly, if the impulse is the same, the impact force decreases as the duration of the collision increases. Therefore, by extending the duration of the collision to reduce the impact force to a level below what the object can withstand, the object can remain intact without breaking.
How do cushions and airbags reduce the impact?
This principle is easy to understand if you imagine dropping a bottle onto a concrete floor and onto a cushion, respectively. If a bottle is dropped from the same height, its speed just before hitting the ground is the same, so the impulse is also the same. However, because the concrete floor is very hard, the collision time is very short, whereas the cushion compresses, lengthening the collision time. As the duration of the impact increases, the impact force decreases; therefore, a bottle dropped onto a cushion is much less likely to break. Consequently, filling a box with cushioning materials such as paper or yarn allows the cushioning material to compress, extending the duration of the impact and enabling the cushioning material to safely protect the bottle.
Car airbags operate on the same principle. If the impact of a collision were transmitted directly to the driver, the driver could suffer serious injuries from slamming into the windshield or steering wheel. To prevent this, the car deploys the airbag within a very short time immediately after the collision. Since the driver collides with the soft airbag rather than a hard structure, the collision time is prolonged, and as a result, the impact force transmitted to the body is significantly reduced.
So, does that mean the more air an airbag contains, the better it is? Not necessarily. If there is too much air, the airbag becomes too rigid, preventing the impact duration from increasing sufficiently. Therefore, airbags are designed with the appropriate pressure and structure to allow them to deform sufficiently to absorb the impact while preventing the driver from coming into contact with the vehicle’s interior structure.
How does mechanical engineering protect our safety?
In addition, technologies such as incorporating shock-absorbing materials inside car bumpers, using corrugated cardboard for packaging boxes, filling snack bags with nitrogen, and installing shock-absorbing flooring under children’s playgrounds are all designed to extend the duration of impact and reduce the force of the impact. As such, although we may not readily notice them in our daily lives, various shock-absorbing technologies are utilized to ensure convenience and safety, and these technologies are the result of research and experimentation in mechanical engineering.
