# Rear\-End Collision Physics How momentum passes from a rolling car into a stopped one in a low\-speed rear\-end bump, and how the seat, head restraint and neck respond\. By Larab Mohsin Topic: Physics & Chemistry [Open the interactive Jig](https://ask.jig.com/j/Rn9snwO74AH2) ## Transcript ### 1. A bump from behind A rear\-end collision happens when one car runs into the back of another\. Here an orange car rolls into a stopped blue car at about 10 km/h, roughly a fast jog\. Watch it again in slow motion, and drag anywhere to look around\. Every number here comes from an illustrative model, not a forensic calculation for any real crash\. ### 2. Momentum moves forward Momentum is the 'oomph' a moving thing carries: its mass times its speed\. During the bump, the orange car hands momentum to the blue car\. In the bar above, orange shrinks as blue grows, while the total stays almost the same\. Illustrative: two cars of about 1,400 kg each\. Momentum is conserved during the brief push\. ### 3. Crumple, then spring back Bumpers, foam and metal squash for a moment, then partly spring back\. Squashing takes time, and that matters: the push is spread over a longer moment instead of arriving all at once\. The crush is exaggerated so you can see it\. Small dents can still mean a real change in speed\. ### 4. Impulse, delta\-v and acceleration Impulse is the total push: force multiplied by how long it lasts\. Here it changes the blue car's speed by about 7 km/h, a change called delta\-v\. Acceleration is how quickly speed changes\. Pick a pulse length and compare the curves\. Each curve shows acceleration \(height\) over time \(length\); 1 g is the pull of Earth's gravity\. Same delta\-v means the same area under every curve, so a longer pulse is lower and gentler\. ### 5. Inside, the body moves first In this cutaway view, with the side of the car removed, the seat back pushes the body forward first\. The head, resting on its own, lags behind for a split second\. The seat belt keeps the hips in place, but in a hit from behind the seat back does most of the work\. The crash dummy and seat are simplified, illustrative shapes\. ### 6. The neck's S\-curve For an instant the lower neck bends back while the upper neck tips forward, making an S shape\. Then the head tips back until the head restraint catches it\. Finally everything rebounds forward and the belt catches the chest\. The neck has seven small bones called vertebrae\. Amber ones are bending backward; teal ones are bending forward\. ### 7. Where you sit matters A head restraint helps most when it sits close behind the head and about as high as its top\. Lean forward and the head travels farther before it is caught, so the neck bends more\. Compare the two positions\. Safety tests rate a gap of about 7 cm or less as good\. ### 8. Speed alone can't tell Two bumps at the same speed can feel very different inside\. Delta\-v, pulse length, the cars' masses, the seat, the head restraint, posture and each person's own body all play a part\. That is why collision speed alone cannot show whether anyone was hurt\. This is an illustrative teaching model\. It is not a case\-specific forensic calculation\.