1. PF Contest - Win "Conquering the Physics GRE" book! Click Here to Enter
    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!

Centripetal force/friction on a car in circular path

  1. May 10, 2013 #1
    I've read so much stuff on the internet including other threads on the forum but am still confused. When a car is making a turn on a level road, WHY does the static friction point inward??? From what I've learned the direction of static friction points in the direction opposing movement but the tires are not moving outward so how can static friction point inward? Please help I really want to understand this before I move on.

    Also, in a rotating cylinder with no floor is the centripetal force the normal force? And do things stick to the wall because of inertia?

    thank you so much
  2. jcsd
  3. May 10, 2013 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    For the Car;
    The net unbalanced force has to point in the direction of the acceleration.
    For the situation described, what is the direction of the net unbalanced force?
    Note: what motion does static friction oppose?

    For the cylinder;
    Yep; pretty much: things want to go straight but the wall keeps pushing them towards the center.
  4. May 10, 2013 #3
    Thanks for the reply!

    Well the net unbalanced force is pointed toward the center but from what I've read, centripetal force does not have an outward force? I don't understand how friction can be pointing to the center if there is no force at all pointing away from the center. I'm so confused.

    Also for the cylinder I understand how an object would stay attached to the wall if the cylinder started to spin when the object is already at the wall but what if an object is just sitting still away from the cylinder when it starts to spin, will it still stick to the wall? And why?

    I know these are a lot of questions any help would be awesome.
  5. May 10, 2013 #4


    User Avatar
    Science Advisor
    Gold Member

    Friction is the centripetal force(Fc) here.
    With Fcs remember that there must always be an actual physical force performing this function. Fc is just a value for that force that is necessary for circular motion. It never exists by itself.
    For example, the tension of a string when swinging a weight on the string around your head; the push of the walls of the rotating cylinder from your second question; the force of gravity for orbiting bodies; the friction on the car tires when turning. These are all the forces that may preform the function of Fc.

    Look at the system from outside. Let's say it's in space, so there's no gravity to worry about.

    You've got these rotating walls of the cylinder, and a motionless object in its centre. For the object to start moving, some force has to act on it, otherwise it'll stay as it is(Newton's first law of motion).
    If we're in vacuum, then there's nothing really to impart such force upon the object.
    If there's air filling the cylinder, then the rotating walls will hit the air molecules touching them causing them to move along with the cylinder(friction). These molecules will hit those higher up(where up is, of course, towards the axis of rotation, where our object sits), and those will hit those even higher, finally hitting the central object. Since this is happening simultainously from all sides of the objects, it begins to rotate in the same direction as the cylinder does.

    In the idealised situation this would be the end of it, the object rotating in the centre of the cylinder. But a slightest turbulence, uneven momentary distribution of air molecules, will mean that at one moment there's more molecules hitting the object from one side than the other, pushing it out of the spot in the centre of rotation.

    Now the object has got some velocity, which is only exaggerated by the continuous assault of air molecules. It drifts in a spiral towards the walls, where it hits the surface and is accelerated to the final tangential velocity by the friction with the walls.
  6. May 10, 2013 #5

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper

    Just take it one step at a time.
    You've established that the unbalanced force must point to the center.
    The only place that force comes from is the contact of the tires with the road - therefore the force has to come from friction.
    That's just deduction.

    Your trouble is that you are used to friction opposing motion ... in this case, the car would like to move away from the center of the turn - just see what happens when you remove the friction! That is the motion that is being opposed.

    It it was a train going in a circle on rails, you wouldn't have a problem right?
    What do you think happens when you turn the front wheels?

    No - all other things remaining equal, it will just stay there as the cylinder spins around it. I think Bandersnatch has covered this one pretty well.

    It's an important distinction when you want to use rotation for gravity in a big space-habitat like in Titan (Varley J.) or Babylon 5 (Straczynski J. M.).
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook