Physics
Impact Force
Quick fact
The impact force during a car crash can be reduced by increasing the time over which the collision occurs, which is why airbags and crumple zones save lives.
Why this is interesting
Have you ever wondered why a glass shatters when dropped on concrete but survives on carpet? The answer lies in impact force.
Read the full explanation
Understanding Impact Force
Impact force is the force that arises when two objects collide. It is not a fixed value but depends on how quickly the collision happens. Imagine catching an egg: if you keep your hands stiff, the egg breaks; if you move your hands backward with the egg, you increase the time of impact, reducing the force. In physics, force equals the change in momentum divided by the time over which that change occurs. When an object hits a surface, it rapidly decelerates—its momentum changes from high to zero in a tiny time window. That short time creates a large force. Surfaces like concrete offer almost no give, so the time is extremely short, leading to huge forces. Soft surfaces like carpet allow the object to sink in, extending the time and reducing the force. This explains why a glass survives on carpet but not on concrete.
A deeper explanation
The underlying principle is the impulse-momentum theorem: impulse (force multiplied by time) equals change in momentum. During a collision, the force is not constant, but the average impact force can be calculated as the total change in momentum divided by the duration of contact. This reveals why time is so critical. In a car crash, the passenger's momentum changes from high speed to zero. Without safety features, that change happens in milliseconds, creating enormous forces. Airbags and crumple zones intentionally increase the collision time—airbags by inflating and deforming, crumple zones by collapsing—thereby reducing the average force on occupants. Similarly, in sports, a boxer's glove or a cricket pad increases the time of impact to lessen injury. Impact force also governs the design of helmets, flooring in playgrounds, and even the way martial artists break boards (by applying force quickly and over a short time). Understanding the mechanism allows engineers to optimize materials and structures for safety.