# Friends discussion: To brake or not to brake?

Hi there,

we are having a tough discussion among friends concerning this topic:

Imagine you are inside your car, stopped in the road. There is no obstacles in front of you, just plain road.
Suddenly, because of the sound coming from the back, you realize another vehicle is going to impact you at the rear.
Would you break or not? Why?

I decide not to brake to release part of the energy of the impact thanks to the more "rolling-free" movement of my car. This will also lead to get less damage/deformation in both cars.

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A.T.
This will also lead to get less damage/deformation in both cars.
Are you trying to minimize the damage to the car, or to the people in the car? If the later, how are the people secured? Do they have head restraints to prevent whiplash?

Priority is to minimize damage to people inside car. Let´s assume all ocupants are properly seated, with seat belts on and modern head restraints systems.

A.T.
Priority is to minimize damage to people inside car. Let´s assume all ocupants are properly seated, with seat belts on and modern head restraints systems.
If everyone is well fixed in the car, not breaking seems the better choice. Although I don't know how much difference it would actually make for the damage.

FactChecker
Gold Member
Braking makes the car a more sturdy, secure barrier against the other car. That will decrease the acceleration that you feel.

On the other hand, if the other car crushes your car enough to actually hit your body, that would be bad. So that is a case where braking might hurt since more acceleration might avoid your body being squashed.

So perhaps the best thing would be if the back brakes were applied, decreasing your acceleration the forward motion of the back, and the front brakes were not applied, so you are not squashed into the front.

Staff Emeritus
2019 Award
I have been in that situation, parked with the engine not started. What I did was to place the car in neutral, and once I was moving begin to brake.

The idea is to put as much of the energy transferred into kinetic energy, rather than into deforming the cars and the passengers inside.

davenn
marcusl
Gold Member
A whiplash injury is caused by the head snapping backwards--the car seat pushes your torso forward while inertia causes the head to lag behind (it wants to stay stationary). The goal is to keep your car and seat from moving forward. Applying the brakes hard, having sticky tires (snow tires if it's slippery), and having a massive vehicle to minimize acceleration, give the best outcome.

Note that even if your head is against the headrest so it accelerates with your body, your brain (which floats in fluid) will stil lag by inertia and will impact the skull in a severe collision. That's a concussion. The optimal is to prevent your car from moving.

EDIT: Deforming the car's crumple zones is desirable when looked at from this perspective. Crumpling dissipates energy while keeping the passenger compartment from moving forward. You definitely want crumpling.

The motion forward does not causes whiplash. The acceleration does.
If your car moves very easily forward, the force (and so the acceleration) will be reduced, as described by Vanadium above.
Even better will be if you can be already moving forward when hit from behind.

marcusl
Gold Member
Assume your vehicle is at rest. How can having it freely moving minimize its acceleration?

A.T.
The motion forward does not causes whiplash. The acceleration does.
If your car moves very easily forward, the force (and so the acceleration) will be reduced
Nonsense. If the car is easy to move forward (no brakes, idle gear) it will accelerate faster, because there is less force opposing the push from behind.

A.T.
Note that even if your head is against the headrest so it accelerates with your body, your brain (which floats in fluid) will stil lag by inertia and will impact the skull in a severe collision.
The problem is not so much brain impacting the skull (as it floats in a fluid of similar density), but rather rotational movement of the brain, relative to the skull, which can sever blood vessels. If your head is at the head rest it won't rotate much upon impact. But it's very hard to say what the optimum here is generally. It might depend on the cars involved, impact speeds etc. And in a real world situation moving forward might mean hitting another car or being hit by cross traffic on a crossing.

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Merlin3189
Homework Helper
Gold Member
...
If your car moves very easily forward, the force (and so the acceleration) will be reduced, ...
I don't agree that the accn is reduced. The easier it is to move, the greater the acceleration produced by any force.
If the car moves easily, a smaller force is needed to produce a given level of acceleration.
But that force is the force applied to the car (including you), not the force applied (by the car) to you, which is the important one.
That force depends only on your mass and your acceleration (which, I think, is going to be close to that of the car.)
And you minimise the acceleration, as marcusl said, by having the maximum force opposing the acceleration.

marcusl
Gold Member
Mercedes-Bnz was the first company to introduce an automatic rear-end active safety system. When it detects an imminent rear-end collision, it applies the brakes at 40%, increasing to 100% shortly before impact. To repeat, anything that prevents or lessens motion (acceleration) of the passenger compartment will lessen injuries.

billy_joule
A.T.
Mercedes-Bnz was the first company to introduce an automatic rear-end active safety system. When it detects an imminent rear-end collision, it applies the brakes at 40%, increasing to 100% shortly before impact.
Makes perfect sense, because in general:
- Rolling forward after the impact might cause more mayhem.
So that should be the real world strategy. However, the OP was asking about ideally fixated passengers.

I don't agree that the accn is reduced. The easier it is to move, the greater the acceleration produced by any force.
If the car moves easily, a smaller force is needed to produce a given level of acceleration.
But that force is the force applied to the car (including you), not the force applied (by the car) to you, which is the important one.
That force depends only on your mass and your acceleration (which, I think, is going to be close to that of the car.)
And you minimise the acceleration, as marcusl said, by having the maximum force opposing the acceleration.
Yes, but in analyzing various cases you cannot assume that the force on the car is the same in all cases.
As you say, a smaller force will be needed to accelerate the car until it moves as fast as the first car and the force will not increase after that.
The force only increases as long as the (initially) stationary car moves slower than the other car.
But the force depends also on how easily it deforms.
I think the car will be less damaged if it's free to move but I did not mean it will be the best for passengers.

The outcome may depend even on the passenger expecting or not the collisions.
Especially that is not all about whiplash (strain in the neck muscle) but also what is called traumatic brain injury.
If you know that he's coming I suppose you can tighten our muscles and lay back against the chair.

256bits
Gold Member
Mercedes-Bnz was the first company to introduce an automatic rear-end active safety system. When it detects an imminent rear-end collision, it applies the brakes at 40%, increasing to 100% shortly before impact. To repeat, anything that prevents or lessens motion (acceleration) of the passenger compartment will lessen injuries.
Such a system will be useful if a similar size or smaller vehicle is the one doing the imminent rear-end collision.
The occupants of the front car will perhaps suffer less injury.
Can the same be said for the imminent rear-end collision vehicle occupants? Hmm.

I would not care to have the system activated if a much larger ( heavier ) was the rear-end collision vehicle. I would, I think, rather be accelerated forward than pancaked.