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The title is a framing for a theoretical question; I'm not asking for practical advice. A friend was recently in this situation and my attempts to apply what I remember of a couple semesters of co...
Question
classical-mechanics
#2: Post edited
by
Monica Cellio
·
2022-09-15T14:49:50Z (about 2 years ago)
added something about speed (raised in comments)
- The title is a framing for a theoretical question; I'm not asking for practical advice. A friend was recently in this situation and my attempts to apply what I remember of a couple semesters of college physics were inconclusive.
- Suppose you are in a stopped car. You notice another car behind you moving too quickly to stop before hitting you. Your goal is to minimize your injury. Strictly from a physics perspective of these two cars, are you better off braking hard or releasing the brake so you'll be pushed? My gut feeling (assuming no one's in front of you) is that you should do the latter, but my friend and I got into a conversation about elastic and inelastic collisions and the effects of friction and of the masses of the two vehicles (if not similar), and now neither one of us knows how to answer the question with physics rather than with gut feels.
In this collision, some force will be absorbed by the car being hit, some will be absorbed by the car doing the hitting, some will go into forward momentum (the front car being pushed), and some will be lost to friction. Some of the force that goes into the car being hit will be transmitted to the occupants. How should I think about these forces and their distribution in the two scenarios (braking and not braking)? And how do the relative masses of the vehicles affect the outcome? A car hitting a car seems different from a motorcycle hitting a semi (or vice versa), but when does a less-extreme difference still matter?
- The title is a framing for a theoretical question; I'm not asking for practical advice. A friend was recently in this situation and my attempts to apply what I remember of a couple semesters of college physics were inconclusive.
- Suppose you are in a stopped car. You notice another car behind you moving too quickly to stop before hitting you. Your goal is to minimize your injury. Strictly from a physics perspective of these two cars, are you better off braking hard or releasing the brake so you'll be pushed? My gut feeling (assuming no one's in front of you) is that you should do the latter, but my friend and I got into a conversation about elastic and inelastic collisions and the effects of friction and of the masses of the two vehicles (if not similar), and now neither one of us knows how to answer the question with physics rather than with gut feels.
- A comment pointed out that the speed of the moving car is important. Assume the stopped car is in city traffic (say, stopped at a traffic signal) and the car bearing down on it is going about 25mph.
- In this collision, some force will be absorbed by the car being hit, some will be absorbed by the car doing the hitting, some will go into forward momentum (the front car being pushed), and some will be lost to friction. Some of the force that goes into the car being hit will be transmitted to the occupants and some to the car itself (crumple zones etc). How should I think about these forces and their distribution in the two scenarios (braking and not braking)? And how do the relative masses of the vehicles affect the outcome? A car hitting a car seems different from a motorcycle hitting a semi (or vice versa), but when does a less-extreme difference still matter?
#1: Initial revision
by
Monica Cellio
·
2022-09-14T23:37:17Z (about 2 years ago)
If you're stopped and about to be hit from behind, should you brake or release the brake?
The title is a framing for a theoretical question; I'm not asking for practical advice. A friend was recently in this situation and my attempts to apply what I remember of a couple semesters of college physics were inconclusive. Suppose you are in a stopped car. You notice another car behind you moving too quickly to stop before hitting you. Your goal is to minimize your injury. Strictly from a physics perspective of these two cars, are you better off braking hard or releasing the brake so you'll be pushed? My gut feeling (assuming no one's in front of you) is that you should do the latter, but my friend and I got into a conversation about elastic and inelastic collisions and the effects of friction and of the masses of the two vehicles (if not similar), and now neither one of us knows how to answer the question with physics rather than with gut feels. In this collision, some force will be absorbed by the car being hit, some will be absorbed by the car doing the hitting, some will go into forward momentum (the front car being pushed), and some will be lost to friction. Some of the force that goes into the car being hit will be transmitted to the occupants. How should I think about these forces and their distribution in the two scenarios (braking and not braking)? And how do the relative masses of the vehicles affect the outcome? A car hitting a car seems different from a motorcycle hitting a semi (or vice versa), but when does a less-extreme difference still matter?