1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Forces, collisions and pain

  1. Jun 23, 2013 #1
    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?
  2. jcsd
  3. Jun 23, 2013 #2


    User Avatar
    Science Advisor
    Homework Helper

    Hi ViolentCorpse! :smile:
    But the first person is hit by the car at 0 mph …

    why should that hurt? :confused:
  4. Jun 23, 2013 #3
    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.
  5. Jun 23, 2013 #4


    User Avatar
    Science Advisor
    Homework Helper

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

    it's a lot less than g …

    and g doesn't hurt, does it? :wink:
  6. Jun 23, 2013 #5
    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?
  7. Jun 23, 2013 #6


    User Avatar
    Science Advisor
    Homework Helper

    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)
  8. Jun 23, 2013 #7
    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.
  9. Jun 24, 2013 #8
    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?
  10. Jun 24, 2013 #9


    User Avatar
    Science Advisor
    Homework Helper

    Hi ViolentCorpse! :smile:
    I think you're boasting

    I bet you can't feel your clothes! :wink:
  11. Jun 24, 2013 #10
    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?
  12. Jun 24, 2013 #11


    User Avatar
    Science Advisor
    Gold Member

    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.
    Last edited: Jun 24, 2013
  13. Jun 24, 2013 #12
    So nicely explained! Thank you so much, Bandersnatch!

    That's a load off my mind. =)
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook