Why do collisions cause more pain than forced movement?

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    Collisions Forces Pain
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Discussion Overview

The discussion revolves around the question of why collisions seem to cause more pain than forced movement, particularly in the context of a hypothetical scenario involving a person tied to a car. Participants explore the differences in forces experienced during a collision versus gradual acceleration, touching on concepts of acceleration, fictitious forces, and the nature of non-contact forces like gravity.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • Some participants propose that a person tied to a moving car would not feel much force and would not be hurt as badly compared to being hit by a car at speed.
  • Others question the reasoning behind the perceived lack of pain during gradual acceleration, suggesting that fictitious forces might still be present.
  • A participant notes that while fictitious forces exist, they are relatively small and not typically harmful, unlike the forces experienced during a collision.
  • There is a discussion about whether fictitious forces can become dangerous, with one participant citing examples from aviation where rapid acceleration can lead to unconsciousness due to "g-forces."
  • Another participant emphasizes the difference in acceleration rates between gradual movement and a collision, arguing that the force experienced during a collision is significantly greater due to the rapid change in speed.
  • One participant acknowledges the hypothetical nature of the scenario and expresses curiosity about the effects of equal forces in both situations.
  • Participants also delve into the nature of gravity as a non-contact force, discussing why it is not felt in the same way as physical contact forces.
  • Another participant explains that gravity does not cause deformation of the body uniformly, which is why it is not directly felt, contrasting it with the localized forces experienced during contact.

Areas of Agreement / Disagreement

Participants express differing views on the effects of collisions versus gradual movement, with no consensus reached on the initial question. There is also a lack of agreement on the nature of forces experienced during acceleration and the perception of gravity.

Contextual Notes

The discussion includes hypothetical scenarios and relies on assumptions about forces and accelerations that may not be universally applicable. The nuances of how forces are felt and perceived are explored without definitive conclusions.

ViolentCorpse
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So this question I have is so obvious that it's probably not even worth talking about from a scientific point of view but I just can't keep it out of my head. The moderators are welcome to delete this if they think it's too silly.

Suppose there is a person tied to the windshield of a car. When the car starts moving, the person wouldn't feel much force from it and wouldn't be hurt that bad. However, if the person was standing still on the road and the car collided with him with the same force, he's going to be hurt pretty badly.

Of course I don't have any actual experience with such a situation and I can not say that tying a person to a car and moving them around won't hurt the same, but intuitively I feel it won't.

So if I am right in my conjecture, why is a collision more dangerous if the forces involved are the same?
 
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Hi ViolentCorpse! :smile:
ViolentCorpse said:
Suppose there is a person tied to the windshield of a car. When the car starts moving, the person wouldn't feel much force from it and wouldn't be hurt that bad. However, if the person was standing still on the road and the car collided with him with the same force, he's going to be hurt pretty badly.

But the first person is hit by the car at 0 mph …

why should that hurt? :confused:​
 
Hi tiny-tim! :)

I take it that you mean to say that the car and the person are in the same frame of reference, so they're not moving with respect to one another? Hmm. But aren't there still going to be fictitious forces present if there is an acceleration involved?

I'm sorry but I'm terrible at physics.
 
ViolentCorpse said:
But aren't there still going to be fictitious forces present if there is an acceleration involved?

yes, but it's a pretty small fictitious force!

it's a lot less than g …

and g doesn't hurt, does it? :wink:​
 
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Haha, of course not. =)

Though, I'm tempted to ask: Is it possible (at least theoretically) for that fictitious force to be so large as to be potentially dangerous?
 
ViolentCorpse said:
Is it possible (at least theoretically) for that fictitious force to be so large as to be potentially dangerous?

Yes, that's what happens when a spaceship or an aeroplane accelerates too fast …

the pilot experiences "g-forces" that cause unconsciousness.

(But they won't break any bones, since the "g-forces" act equally along the whole bone, unlike a car collision, where the bone is usually hit in one specific place)
 
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ViolentCorpse said:
Suppose there is a person tied to the windshield of a car. When the car starts moving, the person wouldn't feel much force from it and wouldn't be hurt that bad. However, if the person was standing still on the road and the car collided with him with the same force, he's going to be hurt pretty badly.

I bolded your error in reasoning.

Force=mass x acceleration
In the first case the person may be accelerated from 0 to 30 mph over the course of 30 seconds so the acceleration would be 1 mph per second. In the second case the car is already going 30 mph and does not slow significantly as it hits the person. The person accelerates from 0 to 30 in something like 0.01 seconds. Since the change in speed happens 3000 times faster the acceleration is 3000 times greater. Therefore the force involved is 3000 times greater.
 
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mrspeedybob, I was aware of that but I'm guilty of constructing a highly hypothetical scenario. The way you have described it is how things will happen in the real world, but I was just wondering whether the person in case 1 would be hurt if the force somehow happened to be as strong as in case 2.
tiny-tim has answered that, though.

Thanks a lot for your response gentlemen!

I have another similar question: Why don't we feel the pull of gravity? "Feel" as in we feel a touch? I know that it is a very weak force, but we can feel the slightest touch of a finger, so why not gravity's pull? Or in more general terms: Can we feel a non-contact force like we feel physical contact forces?
 
Hi ViolentCorpse! :smile:
ViolentCorpse said:
… we can feel the slightest touch of a finger …

I think you're boasting …

I bet you can't feel your clothes! :wink:​
 
  • #10
tiny-tim said:
Hi ViolentCorpse! :smile:


I think you're boasting …

I bet you can't feel your clothes! :wink:​

Hello tiny-tim! :smile:

Haha. Not in the summer, no. :biggrin:

Is Earth's gravitational force really even weaker than that of a palpable touch?
 
  • #11
Earth's gravitational force on your body is your mass in kilograms(comonly called "weight") times 9,81 m/s^2. That's in the ballpark of 800 Newtons for an average male person. It's not an insignificant amount, as can be experienced by having some random bloke sit on you.

The force of gravity is, however, not experienced directly due to it being (locally)uniformly permeating space around us. For a body to "feel" a force, it has to deform the body by accelerating some parts of it faster than the other.
When somebody touches you, the force(which is not really a contact force per se, as on the atomic scale it's just the Lorentz force between electrons) is applied locally, deforming your body and trigerring sensory response.
Gravity, if sufficiently far away from the source, accelerates all parts of your body equally, producing no deformation, hence no sensory response here.

Of course, as we get accelerated by gravity, we tend to come in contact with the ground, which pushes on our feet(or whatever) with force equal, but opposite, to gravity. This push by the ground is localised, so we can feel it. It's also what most people would call "feeling gravity", even though it's not technically correct.

In certain cases the force of gravity can be experienced. You need to be close enough to the source that your size becomes comparable to your distance from it. The actual ratio is dependent on the mass of the source. Higher mass means the farther away you can be to notice the effect. The Moon gets deformed in Earth's low gravitational field due to it's closeness and size. A man would be appreciably deformed e.g., when falling into a black hole.
These are called tidal forces, and they tend to stretch your body along one axis and compress it along the other two. This is obviously non-uniform application of the force, so it would be felt.
 
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  • #12
So nicely explained! Thank you so much, Bandersnatch!

That's a load off my mind. =)
 

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