Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Does friction increases as speed increases?

  1. Dec 31, 2008 #1
    hi, I would like to know does friction increases as speed increase.(all factors like surface of the road are held the same). For example, if I am on the road at 0m/s then I applied a constant power to my car via the fuel pedal, base on Newton's 3 laws, I would be accelerating, but I guess the friction must also increased right? Because if it doesn't, it will accelerate forever(getting faster and faster).
  2. jcsd
  3. Dec 31, 2008 #2

    Ranger Mike

    User Avatar
    Science Advisor

    you are pushing aside a massive curtain of AIR when moving. Stick your hand out the window at 20 MPH..try it at 65 MPH..you will notice a slight difference..this is not readily apparent since we walk around in it at ground level but it is 14.7 psi reality..that is why two cars ' drafting' can go faster than one car ..at high speeds i.e + 160 MPH
  4. Dec 31, 2008 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    There is a difference between friction and fluid resistance. The deformation of the car tryes and the road surface results in the so-called rolling resistance between the tyres and the road, which is independent of velocity (i.e. the rolling resistance remains constant regardless of your speed).

    However, as Ranger Mike points the car also experience fluid resistance or drag. For large objects travelling at high speeds the magnitude of the drag force is considered to be proportional to the square of the velocity of the car.
  5. Dec 31, 2008 #4

    Ranger Mike

    User Avatar
    Science Advisor

    thanks Hootenanny..my education continues..always wondered technical name for that
  6. Dec 31, 2008 #5
    I am not so sure rolling reistance for an automobile should be considered a constant over a wide range of speeds. There are a number of factors at work inlcuding the aerodynamic shape of the car which may increase the force against the road (or reduce it) and the
    effects of heat with speed as well. Racing teams heat their tires, spin them in place, to improve traction...

    See wikipeida http://en.wikipedia.org/wiki/Rolling_resistance
  7. Dec 31, 2008 #6

    Funny, I was thinking about this recently. Can someone point me to an explanation of why the force goes as velocity squared? I saw something that says at low speeds (laminar) the friction is linear with velocity, but in turbulent conditions it is velocity squared. Is that right? Whats the physical explanation for that?

  8. Dec 31, 2008 #7


    User Avatar
    Science Advisor
    Homework Helper
    Gold Member

    Whether the drag is linear or quadratic is a fuinction of the Reynold's Number; generally speaking, the drag force is linear at very low speeds, and quadratic (increases as the square of the speed) at higher speeds. Regarding also your question as to whether friction increases at increasing speed, it was noted in the above posts that yes, the air drag increases (air resistance), but also the static friction force between the driving tires and the road also increases at increasing speed. The friction between the road and drive wheel tires is what propels the car forward; It is equal to (u_s)N at max speed without the tires slipping, but it is less than (u_s)N at lower speeds (as you noted, it is 0 when the car is not moving!).
  9. Dec 31, 2008 #8

    Ranger Mike

    User Avatar
    Science Advisor

    almost correct..teams use tire warmers to bring them to optimum temps prior to racing...thus max traction capability..the in place tire spinning is Heat Cycling..this process "Scuffs" the tires in under controled conditions as opposed to running the new Sticker tires ( they still have manufacturers stickers on the tread) for a few laps..the best driver in the world will still heat shock a new tire by " leaning " on them for a lap or two and heating them over correct tire temp for optimum life.
    Last edited: Dec 31, 2008
  10. Dec 31, 2008 #9

    So that is to say if we are in a vacuum(no air), we will accelerated forever as the friction between the tires and the car is constant and no fluid resistance/drag(vacuum).
  11. Jan 1, 2009 #10

    Ranger Mike

    User Avatar
    Science Advisor

    If you have a propulsion unit available , that would provide infinite ACCELERATION, then the above scenario would be the case, I guess. I know of no such unit. All engines I know about accelerate to a maximum point and either top out at max RPM or sufffer catastophic mechanical failure (grenade). Besides tire to pavement friction there is a lot of internal parasitic drag going on. Converting chemical power to thermal power, i.e. the internal combustion process ( otto cycle) requires much mechanical engineering. Reciprocating to rotary motion conversion through linear piston motion /crank shaft, some type transmission and differential ..all have mucho friction that only increases under load (acceleration)..things heat up when you do big time acceleration.

    When I was a kid at the drag strip, a Super Stock Dodge sawed a drive shaft at 120 MPH in the traps ( end of the 1/4 mile)
    Wanting a piece of a real race car to take home I stupidly picked up the universal joint piece..and dropped it..ouch!!
    an old drag racing guy saw me do it..said " Hot isn't it, sonny!"
    "No" I replied, "just doesn't take me all day to look at it!"
    I was a wise A-- back then too...it was very HOT!

    Comparing chassis dynometer horsepower to actual engine dyno numbers tell us we loose 150 Hp between the engine and the tires...
    just more to think about on New Years day..
    Last edited: Jan 1, 2009
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?