Direction of Friction!

  1. Hi guys
    Consider a wheel rolling up the hill by some external force. What will be the direction of friction?
    I know it's a basic question but i got confused last night with friction in rotation and rolling.
  2. jcsd
  3. The direction of friction would have to be down the hill in order for the wheel to roll in the correct direction.
  4. if i give it a force then it's ok i understand u or myself
    but consider an axle of a car , it gives a wheel a torque , so in this case torque has already been provided so will friction act up the hill?
  5. My original logic in this post was incorrect, so it is deleted (12/06/09)!

    A diagram is included with a comment below.
    Last edited: Dec 6, 2009
  6. but the reaction force is friction. Isn't it?
  7. Doc Al

    Staff: Mentor

    Yes. The friction of the road will act up the hill thus propelling the car up the hill.
  8. in this case why will car propell up the hill?
    If i apply external force to a wheel , friction will act downwars providing the wheel with torque, which will make it rotate and the linear force i provided combining with the rotation will create a "roll".
    But in above case torque is provided b axle it's ok but what about linear component of force required too create a roll. If u say it's friction I say that frition will just give an anti torque i.e just trying to stopp the rotation of wheel. What will give the wheel a linear component??Bcoz linear component together with rotation produces 'roll'.
  9. Doc Al

    Staff: Mentor

    Friction from the road provides a linear force as well as a torque.
  10. Why will friction provide a linear force. Actually friction provides a linear force a radius distance apart from center of rotation which creates a torque(a torque is always a couple so in this case one force is at the top of wheel due to my push and other and opp is due to friction).
    What propells it forward?
  11. russ_watters

    Staff: Mentor

    Both. The forces come in pairs, otherwise the wheel would freely accelerate when you apply a force to it.
  12. Why would wheel freely acc when i apply force to it? When i would give it force , the force will be on all points on it and also at the top. Now at bottom friction will give an opp force , the couple creates a torque and this would produce rotation but since a linear force was present at all other points too(my force) it would move forward along with rotation and this is called rolling.
  13. russ_watters

    Staff: Mentor

    F=ma, so if you apply a single force to an object, it will accelerate.
    Right - the engine provides a torque in one direction, the friction with the ground provides a torque in the other direction. In constant speed motion, forces come in pairs (at least), that sum to zero.
  14. Sketch attached. Friction is upward and resolved weight of a cylinder/wheel is downward along the plane angle.

    If no torque acts independently at the axis of rotation, the wheel is rolling downhill, the friction causes a clockwise torque about the axis applying the right hand rule.

    Suppose a second counter-clockwise torque (like at a car axle) acts independently about the central axis. The magnitude of the upward friction force must grow in reaction to this torque to either (a) slow the downward motion; (b) hold the wheel in place on the ramp; or (c) move the wheel up the ramp. I'm fairly certain this description will make sense now.

    Attached Files:

    Last edited: Dec 6, 2009
  15. system theory
    consider the case of a wheel going up the hill.
    Though i understand the case of wheel rolling down, direction of friction clearly.
    What i am asking is : When wheel rolls down firction produces torque and thus rotation and there is a downward motion due to gravity which together with rotation creates a "roll".That's clear .OK.
    But when wheel goes up especially wheel on an axle , torque is provided by axle , friction just counter acts to oppose that torque in uphill direction, then what provides linear motion to wheel to roll instead of rotate.
    In previous case gravity provided linear motion and friction provided rotation and togehter they both created rolling motion.
    But in later case where does linear motion come from?
  16. Doc Al

    Staff: Mentor

    The translational acceleration is determined by both gravity and friction.
    Friction and gravity once again determine the translational acceleration.
    In both cases friction helps determine the linear motion. You seem to be stuck on the idea that friction provides a torque but not a linear force.
    Good idea! :smile:
  17. But friction provides linear force only at the bottom part where wheel touches the ground and that linear force is used for producing torque. Isn't it? Or that linear force also pushes the wheel forward?????:confused:
  18. Doc Al

    Staff: Mentor

    Since friction is applied at a distance from the center, it contributes to the torque on the wheel. But it also contributes to the linear acceleration of the wheel.
    Of course it does. When you use Newton's 2nd law to find the linear acceleration, you must consider all the forces regardless of where they act.
  19. sophiecentaur

    sophiecentaur 13,247
    Science Advisor
    Gold Member

    Would we all be having such a terrible problem is we replaced the wheel / road / drive shaft system by a lever and just considered the first mm of movement? That's all that's happening, really. It's jut that the wheel provides you with a set of levers one of which is always in contact with the road.
  20. Let me say I give a force of 10 Newton uniformly over whole of a wheel. Now every point on the wheel will have an acceleration coresponding to that force or I can say every point will experience a force of 10 N. The bottom point touching the ground will also experience a force and it will apply same force to ground. Now a reactive force or friction will act on the same point in opp direction. Also, there existed same force on the top most point. So a couple will be formed of 10N each force and the wheel will start to rotate. So now 10 N or whole of friction force is used in creating couple, where does linear component come from?

    If u still say friction will give linear component, then relook that force at top point was 10N, so we need a 10N at bottom for rotation to start, so friction provided that 10N which was it's instataneous max. value. So, all 10N used in rotation , where does linear force come from?
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