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

Friciton drives a car forward right?

  1. Mar 12, 2013 #1
    Car wheels spin backwards and the friction force acts forward so the resultant force allows the car to move forward right?

    because I saw someone say something about... cars appear to be spinning backwards on tv.

    But in reality aren't they really spinning backwards?
     
  2. jcsd
  3. Mar 12, 2013 #2

    rcgldr

    User Avatar
    Homework Helper

    There can be an optical illusion with wagon wheels or car wheels with spokes that can make them appear to be rotating backwards, but it's an illusion. The wheels are rotating forward when the car is moving forward.

    For your other question, the tires exert a backwards force onto the pavement and the pavement exerts a forward force onto the tires. Since the earth is massive compared to the car, the end result is that the forward force from the pavement propels the car forwards and that the earth moves "backwards" a very tiny amount.
     
  4. Mar 12, 2013 #3

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Yes - the net force on an accelerating car is forward, it acts at the points of contact between the car and the road (the wheels) and comes, immediately, from friction. More accurately - from the engine driving the wheels against the road surface.
    No - that is an optical illusion caused by the frame-rate of video.
    The edge of the wheel that touches the road is, instantaneously, moving opposite the direction of the cars motion.
     
  5. Mar 12, 2013 #4
    I don't really get that optical illusion.. if the wheels are moving forward shouldn't the car move backwards?
     
  6. Mar 12, 2013 #5

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    If the car is moving left to right, the wheels rotate clockwise (as you are looking at them).

    Get a toy car and play with it. Then go someplace where real cars go slowly and look at what they do.
    Also see:
    http://en.wikipedia.org/wiki/Wagon-wheel_effect
     
  7. Mar 12, 2013 #6
    Well I didn't have a toy car but I observed the wheels on my chair and saw that it went in a clockwise direction.

    Well thanks for the information!

    Wait one more question so that means the car can move forward when it wheels go forward it's just that the car has to overcome the friction or are cars designed with wheels to move backwards?
     
  8. Mar 12, 2013 #7

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    When you walk forwards, your feet push backwards on the ground. The same thing is happening with the wheels. You cannot go forwards without something, in some way, going backwards.

    Probably it will help you to get into the habit of being clear about what you say ... i.e.
    If you follow the hub of the wheel with your eye when you watch it, you'll see that the top of the wheel, at any time, is going in the same direction as the car/chair/whatever and the bottom is always going the opposite direction, while the hub stays still.

    This is hardly surprising because the wheel goes around: that's what makes it a wheel. But it is also a sloppy observation.

    If you look really carefully, you'll see that, as far as the ground is concerned, the bottom of the wheel is stationary(!) and the hub moves over the ground at the same speed as the car while the top goes at twice the speed.

    So it is quite tricky to be consistent about what one means by "forwards" or "backwards" when you talk about a wheel.


    A couple of experiments drive this stuff home.
    Get something round - a dinner plate will work.
    Put it on it's edge against a wall so you can roll it slowly and it is supported.
    Mark a spot on the edge of the plate, put that spot uppermost, and mark the corresponding place on the wall.
    Now roll the plate/wheel forwards a little bit - the spot on the wheel, and the one on the wall are now in different places - mark the new position on the wall.
    Repeat until you have a track of dots on the wall which map out the path of that spot on the edge of the plate as it rolled.
    The result is something like this:
    http://www.math-mate.com/chapter44_3.shtml
    ... but you should do it for yourself or you will never grok it properly.

    Very useful tools for studying physics of motion are
    1. toy cars: matchbox cars or larger Tonka toys if you can get them.
    2. model train set - Hornby or similar, with a length of track. You don't need the engine, but it does help if the wagons can open up.
    3. children's educational building blocks - the kind with the green rollers in them and where all the sides are in simple ratios (i.e. not the alphabet blocks).
    [edit: I'm thinking of "unit blocks"]
    4. assortment of balls, all different sizes and materials - it's hard to have too many.

    The #3 is harder to get hold of but the others can be had real cheap.
    I'm serious - get some.
     
    Last edited: Mar 12, 2013
  9. Mar 12, 2013 #8
    Hmm thanks for telling me all of this! I'll try to get some of those things and study motion more.
     
  10. Mar 12, 2013 #9

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    It will help you intuitive understanding of physics no end :)
    Have fun.
     
  11. Mar 12, 2013 #10

    rcgldr

    User Avatar
    Homework Helper

    Under certain circumstance, you can get that optical illusion from a live view (as viewed by a person), not just from a video or film.

    Wiki article with direct link to the continous illumination section:

    wiki_wagon_wheel_effect.htm
     
  12. Mar 12, 2013 #11

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    @rcgldr: Well spotted: I already posted that link in #2 ;D
    So many people would have just said "tldr" and missed out!
     
  13. Mar 12, 2013 #12

    SteamKing

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    As Yogi Berra would say, "You can see a lot just by lookin'."
     
  14. Mar 13, 2013 #13
    For every action ther is an equally opposed reaction - Sir Isaac Newton... If the car tires are rotating one way the car will go the other way. Sometimes a cars' rims will appear to be going backwards as opposed to the cars' motion. This is an optic illusion caused by the way we process visual motion. We can process about 60 images/second to conceptualize motion...If a wheel is spinning faster than can be percieved in a static state, It might appear to be going 'backwards'...
     
  15. Mar 13, 2013 #14

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    There is not a direct connection between this and the apparent reversal of rotation. TV uses 50 or 60 fields every second and that rate was chosen to fit in with the way our vision works (it is very 'laggy'). TV 'samples' the scene at 60 fields per second because it is fast enough to show most motion OK (film samples at only 24 frames per second - so it's worse at motion portrayal). The result of the sampling process of the TV system (and not our eyes and brains) is that rapidly repeated movements, like the rotation of the spokes of a wheel can be 'under-sampled' and this produces artifacts (called aliases) which can come out as motion reversal. The same thing happens when sound or any other signal is under-sampled - which is why they sample audio at 44kHz when the programme is low pass filtered to 22kHz.
    The aliased moving image, in which the wheels appear to rotate backwards, is a product of the TV system and not our eyes. If the scene were sampled at 120Hz then the wheel could rotate at twice the speed before the aliasing occurred. When you see a High Frame Rate (HFR) movie, you may notice the 'wagon wheel' effect is less frequently seen. You are viewing it with the same pair of eyes.
    The response time of the eye's optical sensors is so slow that 'aliasing' is not likely to occur. The eye's poor response time is, effectively, an anti-aliasing filter.
     
  16. Mar 13, 2013 #15

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    Newtons 3rd Law refers to forces, not movement or rotation. On a car with front-wheel drive all wheels rotate the same way. But the forces at the back and front wheels are opposite.
     
  17. Mar 15, 2013 #16
    Newtown's third law tells us that the earth is acting on the car and thats why it moves, because the equal and oppisite reactionn drives it forward but if it were on a frictionless surface I dont think it would move. And to the guy above me, rotation around a fixed point is called Centripatal FORCE.
     
    Last edited: Mar 15, 2013
  18. Mar 15, 2013 #17
    Rotation around a fixed point is called Centripital FORCE
     
  19. Mar 15, 2013 #18
    But I see it in real life all the time. Am I crazy?
     
  20. Mar 15, 2013 #19

    A.T.

    User Avatar
    Science Advisor
    Gold Member

    No. They have different units.
     
  21. Mar 15, 2013 #20

    rcgldr

    User Avatar
    Homework Helper

    Low pass fiiltered to 20kHz, the sampling rate needs to be more than double, not just double the highest allowed rate, so cd's use an extra 10% beyond double. Most video devices use at least 48kHz.

    I again refer to the "continous illumination" section of that wiki article about "wagon wheel effect":

    wiki_wagon_wheel_effect_our_eyes.htm
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook