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

Weird? About the direction of rotation of wheels

  1. Oct 31, 2012 #1
    Weird!? About the direction of rotation of wheels

    Suppose we consider a car moving on rough road in forward direction, wheels rotating clockwise.
    Since wheels rotate clockwisely they will push road backwards so friction acts forwards i.e. in direction of motion of car.The car will move, because the only external horizontal force on it is the actual friction force.
    Please see the following website:

    From 0:29 to 0:34,it is clear that the left front wheel rotate clockwisely but the car is accelerating forwards

    However,please also see this video,
    http://www.youtube.com/watch?v=VCpPPVvGqTY&feature=related
    From 2:11 - 2:14
    The car accelerates from rest,this time the left front wheel rotates anticlockwisely.

    Why?If the wheel push the road forwards,the road would push the car backwards.

    Moreover,please see the last video,it is about bicycle.

    From 0:30 - 0:40,
    a bicycle wheel would rotate in the same direction of the displacement of bicycle.
    Similarly,why?Shouldn't the wheels rotate in the opposite direction of displacement according to Newton's 3rd law)
     
    Last edited by a moderator: Sep 25, 2014
  2. jcsd
  3. Oct 31, 2012 #2
    Re: Weird!? About the direction of rotation of wheels

    I'm not sure what you're driving at.
    Are you talking about the 'wagon-wheel-turns-backwards' strobe effect of a movie camera?
     
  4. Oct 31, 2012 #3
    Re: Weird!? About the direction of rotation of wheels

    When the car moves forwards,by action and reaction force,the wheel should rotate backwards so a backward force exerts on the ground.A forward force thus exerts on the car.
    Video 1 shows what I mean.
    But in video 2 and 3,the wheel of the car rotates forwards when the car accelerates forwards.This shows a contradiction with video 1?

    Actually I spent nearly half an hour standing on the street this morning and observed whether the wheels rotate forwards or backwards when the car accelerates.OK I am so crazy...
    But I couldn't find any wheel rotate backwards as the car move forwards eventually.All wheels rotate forwards...I am asking because I found it weird and it doesn't match the Newtonian's laws.
    Hope someone can understand what I mean :)
     
  5. Oct 31, 2012 #4
    Re: Weird!? About the direction of rotation of wheels

    I did not see the videos but I think the same as AJ Bentley. Why do you think differently? Or don't you know the stroboscopic effect?
     
  6. Oct 31, 2012 #5

    cjl

    User Avatar

    Re: Weird!? About the direction of rotation of wheels

    Based on looking at the video, I think you're referring to the fact that the wheel appears to be turning backwards when the car is at a certain speed. This isn't because the wheel is actually turning backwards - it is purely an artifact of the way that videocameras work. When the wheel is turning at a certain speed, it aligns a certain way each time the videocamera is taking a picture (to make one frame of the video), which can make the wheels appear to stand still or even turn backwards, even though in reality, they are turning forwards the whole time. A great example of that can be seen here (especially starting at around 1:10):

    http://www.youtube.com/watch?v=YpMtanpsVeE&feature=related

    The fan is rotating clockwise the entire time, but there are periods when the pattern in the middle appears to stop or reverse, and there are periods that the fan blades appear to stop or reverse. Interestingly, since the pattern in the middle repeats 4 times, but the fan has 3 blades, the fan blades are clearly moving while the pattern in the middle appears stopped and vice versa, making it very clear that this is an artifact of the camera.

    This can lead to some especially strange and interesting effects if the camera has a progressive scan, where the entire frame is not exposed at once. Here's an example:
    http://www.youtube.com/watch?NR=1&v=LVwmtwZLG88&feature=fvwp
     
  7. Nov 1, 2012 #6

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Re: Weird!? About the direction of rotation of wheels

    This phenomenon is due to Temporal Sub Sampling. If the frame rate is lower than twice the rate at which spokes pass through a particular angle then an alias is formed which can have a negative angular velocity. This only happens where there are repeated patterns - as with the identical spokes.
    This is a problem in vehicle crash testing labs when they want to assess the wheel speeds. It is resolved by painting a single patch of white on each tyre. This still appears in the film to be going the right way round and at the correct speed until the actual wheel rotation rate exceeds half the frame rate (n times as fast) when the aliasing kicks in again.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Weird? About the direction of rotation of wheels
  1. Rotation of a wheel (Replies: 5)

Loading...