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!

So that's why my car rolls Is it?

  1. Dec 1, 2009 #1
    So that's why my car rolls... Is it???

    Hi, I just wanted to clarify my thoughts so that I can convince my friends of the wonders of classical mechanics...

    The reason that I find it hard to push a car is because of friction, and in the example of pushing a car this is because the atoms of the tyre are bonding, ultimately in terms of electrical and magnetic attractions, with the atoms of the road.

    The reason it's harder to get the car moving with my initial push is that the atoms have a stronger force and as there are more of them due to the compression of the wheel, i.e.the area is slightly larger than the area of the turning wheel so there are more bonds. When the wheel is turning there are less atoms bonding due to the small amount of time they would all be exposed on a level plane (due to the atoms jiggling).

    Would I be able to give that description? Wouldn't this be a good approximation of the reasons for static and kinetic friction? Can you add more or perhaps better reasons for this force.

    That sentence "ultimately in terms of electrical and magnetic attractions" is from my book, if possible, though not essential, can anyone explain that intuitively, maybe the Coulomb equation explains this although I'm not too sure on how.

    Another sentence from my book is " smoothing the surface can actually increase friction, since more molecules are able to interact and bond, " Now I've been told friction is all "Inertial" on yahoo yet my book, "University Physics, with Modern Physics" by Young and Freedman says differently, as in the previous quotes...
  2. jcsd
  3. Dec 1, 2009 #2


    User Avatar

    Staff: Mentor

    Re: So that's why my car rolls... Is it???

    Nope. Has nothing to do with friction between the road and tire. Search on rolling resistance at wikipedia.org or similar.
  4. Dec 1, 2009 #3


    User Avatar

    Staff: Mentor

    Re: So that's why my car rolls... Is it???

    The reason it is harder to get anything moving than to keep it moving is that static friction is higher than dynamic friction. However, with a car, the dynamic friction isn't much lower than the static friction because the car's axles are very well lubricated.

    On a microscopic level, though, yes, there is bonding between atoms of the two surfaces in a friction situation (ehh, edit - but as berke said, it isn't the atoms of the tire and the road, but the atoms of the axle and bearing).

    The reason it is so hard to accelerate a car by pushing on it is the car has a lot of mass and a=f/m.
  5. Dec 1, 2009 #4


    User Avatar
    Science Advisor
    Homework Helper

    Re: So that's why my car rolls... Is it???

    I don't think the contact area changes very much between static and rolling.
    There is also an effect with the rolling wheel that air, water, and stuff is dragged between the wheel and road as it rolls - this lubricates the contact.

    If you put a piece of metal down on another piece of metal - how do the atoms on the surface 'know' which bit of metal they belong to? They wil bond to any atoms in range - whether on their own bit or the other surface.

    That's true for very smooth surfaces. With ultra flat smooth and clean reference flats in vacuum it's possible to contact weld them by simply putting two pieces together. With no air, water, dirt etc between them there is nothing to stop the atoms in one piece bonding with the other piece.
  6. Dec 1, 2009 #5
    Re: So that's why my car rolls... Is it???

    A conceptual sketch is attached to give more insight. The car is at rest. It has mass m and some rotational moment of inertia J in the parts that rotate with the tires. This is shown as one cylinder J.

    The first push must have more force to overcome the static friction of the rotating parts, which is greater than the kinetic friction. The mass m must increase its kinetic energy and so does the rotational inertia J, which requires extra work done by the starting force, to accelerate m and J up to a low speed. This takes some extra starting force.

    The reaction force f appears during acceleration, is smaller than force F, and is necessary to add energy to the rotating parts. When velocity is constant little f becomes much smaller (no more energy is being added to inertia J) and is due mainly to the tire breaking contact with the ground as it rolls.

    The bearing friction in rotating parts is smaller in motion than when starting to push the car. This is simplified, but covers the basics. In a model you don't worry so much about the atomic causes of friction, you measure friction values for a particular system and plug them into the model.

    Attached Files:

  7. Dec 1, 2009 #6
    Re: So that's why my car rolls... Is it???

    I'm well aware of μs & μk but I was just trying to understand why they would be different, visual pictures to describe why there is a difference.
    The air, water etc... sucked under the tyre as it rolls seems very understandable to minimize contact between the atoms of the ground and the tyre. I understand there would be rolling resistance too I suppose I didn't think of all the forces acting.
    I'm just considering the idea from an atomic point of view for a different angle, I know friction is not fully understood (http://memagazine.asme.org/Articles/2009/January/Theres_Rub.cfm) so I was thinking there may be some reason that an intro physics book wouldn't mention lol.

    Thank you for the conceptual sketch too System, illustrates steel scraping off of steel to make the inital push even heavier :p.
  8. Dec 1, 2009 #7
    Re: So that's why my car rolls... Is it???

    As someone mentioned above, the parts are lubricated, which greatly reduces static friction. The tires don't create much resistive friction when you push a car, because they make traction and roll. The reaction force f due to acceleration of the rolling inertia, and the acceleration of mass m with remaining F, explain most of the extra effort required to get the car moving.

    Physics textbooks focus on static and kinetic (dry) friction and these simple concept don't always apply to engineered systems in the same way. Even if there were no friction at all it would take some force to get the car accelerated up to pushing speed!

    That said, if you wish to research atomic friction, it is a reasonable thing to do. The tires are a visco-elastic material and in car racing and performance testing, there is much to learn about these materials.
  9. Dec 2, 2009 #8
    Re: So that's why my car rolls... Is it???

    The car is hard to push cause of newton's 1st law of motion...any mass will resist any change in it's state (by state I mean it's velocity)...the harder you attempt to change it's state, the more will it resist.
  10. Dec 2, 2009 #9
    Re: So that's why my car rolls... Is it???

    The wheel has to do virtually nothing with it...I mean you can even neglect those forces at such a low speed.
  11. Dec 2, 2009 #10


    User Avatar
    Homework Helper

    Re: So that's why my car rolls... Is it???

    If the car has been sitting for a while, then you get a bit of a flat spot on the tread that takes more force to overcome, as it lifts the car a bit when you initially start moving it. Otherwise, it's static friction in the axles and drive train that have to be overcome.
  12. Dec 2, 2009 #11


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    Re: So that's why my car rolls... Is it???

    The tyre is constantly being distorted (flattened) as it rolls along the road - not just after it has been standing still. The engine has to provide a force, effectively, to drive the wheel slightly uphill over the bulge at the front. This involves energy loss as the tyre material is distorted and the tyre gets hot. As you increase the air pressure in the tyre this mechanism reduces. Ideally, the footprint would be vanshingly small (as with steel railway wheels and steel rails) but then traction would a problem.

    I wonder, did anyone ever consider a variable tyre pressure system to reduce rolling resistance when not braking or cornering?
  13. Dec 2, 2009 #12
    Re: So that's why my car rolls... Is it???

    This may be true, I haven't run the numbers in a system model for low speed. However, the starting rate of acceleration, and the rate at which tire spin occurs, is a function of the inertia J in the driveline.

    Also I've simulated the Mythbusters study of big car versus little car racing downhill. The little car (1/64 optimized model) wins for about 50-100 feet, probably due to less inertia J in the driveline relative to its mass m. Starting friction is too hard to estimate, so I left that out.

    Testing of these ideas might be easily accomplished with some small scale models. Getting good measurements without a lab budget is the hard part.

    Edit: here's an interesting discussion of friction that I haven't had time to read:

    http://evolution.skf.com/zino.aspx?articleID=14959 [Broken]

    It turns up on a google image search: lubricated friction

    Edit: an industry education link (I never knew Tribologist's existed!):

    http://www.stle.org/resources/lubelearn/default.aspx? [Broken]
    Last edited by a moderator: May 4, 2017
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook