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Direction of force acting on a body in uniform circular motion?

  1. May 13, 2013 #1
    Direction of force acting on a body in uniform circular motion??

    Hello Sir/Ma'am
    I'm a a novice. I joined this forum to clear my doubts and improve my physics. Here is my question.. " We generally say, frictional force always acts in a direction, opposite to the direction of motion of a body. Is there any case where it acts in a different direction?? For example, if a car is moving around a pole at a uniform speed (uniform circular motion), the friction is acting towards the centre (also called centrepetal force). And we know that the direction of motion of the car at any point is along the tangent i.e perpendicular to the radius. So,
    1) Is it correct to say that here, direction of friction is perpendicular to the direction of motion of the body???
    2) And should i conclude that direction of friction force doesn't always need to be opposite to the direction of motion of the body??
    3) are there any other cases where direction of friction is not opposite to the direction of motion of the body?? Two cases i know of are walking and riding a bike. Any other cases??
    Sir, this is not a homework question. Kindly dont delete it. It is something which i can't understand. Thanks!!
     
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  3. May 13, 2013 #2

    Simon Bridge

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    Welcome to PF;

    YOu should be careful about real world objects like cars - they have lots of forces acting on them - many of them friction.
    The net force in uniform circular motion points to the center (in an inertial frame).
    The only source of this force is the contact between the road and the tires.
    This is friction. Therefore...

    You should conclude whatever you can support with evidence. I won't tell you what to think.

    A block sitting stationary on a slope experiences friction.
    Which direction is the friction?
    Which direction is the motion?

    A car accelerating in a straight line experiences a net frictional force and it is moving.
    What direction does the net friction point in?

    If you think of friction as a kind of stickyness between surfaces, then you can see that it will oppose the motion that would have been there but for the friction.

    Go through each example and see what the motion would have to be without the friction.
     
  4. May 13, 2013 #3
     
  5. May 13, 2013 #4

    Simon Bridge

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  6. May 13, 2013 #5
     
  7. May 14, 2013 #6

    Simon Bridge

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    The wheels only slide backwards in low-friction situations.
    To accelerate, the car must be acted on by an unbalanced force.
    That unbalanced force is friction - it acts at the contact between the wheels and the road and points in the same direction as the acceleration.

    Don't know what you mean by "external force by the steerings" - steering is not a force.
     
  8. May 20, 2013 #7
     
  9. May 20, 2013 #8

    Simon Bridge

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    The net friction in a car going at constant speed is zero.
    There are lots of friction forces.

    Some of the friction forces, such as between the tires and the road, point forward.
    Some of the friction forces, such as at the axles, in the engine, and of the air moving over the body, act to oppose the motion.
     
  10. May 20, 2013 #9
    [REPLY] Thank you Sir for very quick help. You said, ''the net friction in this case is zero.'' But these forces (i.e friction at the axles in the engine and the air friction in the opposite direction) must be acting on the car in the first case also (when the car is accelerating). Hence, the net friction must be zero in that case. Then, what causes the car to accelerate ??
     
  11. May 20, 2013 #10

    Simon Bridge

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    No. Since the different frictions have different causes, there is no reason they must always cancel out.

    Generally the retarding forces are speed dependent - bigger for higher speeds.
    When you supply more power, the driving friction increases, and the speed increases, and the retarding friction increases with the speed until it is equal to the new driving friction. This is what you experience driving your car right? Take your foot off the gas and the driving friction is now less than the retarding friction, the car slows down, the retarding friction reduces until it is equal to the driving friction and you have a new constant speed.
    This is a simplification, true, but it should feel familiar.

    The power to drive the car is supplied by the engine - using energy from the fuel - through a complicated arrangement of machinery which we are modelling using the resulting forces. The forces pushing on the car are contact forces which are by nature friction forces.

    You should be familiar enough with this line of reasoning to be able to come up with your own answers by now - even if you still don't believe it ;)

    The question in front of you is whether friction can ever point in a direction other than the direction of movement. You should now be able to answer that question.
     
  12. May 20, 2013 #11
    Thanks a lot Sir for all those answers.. That really helped. Now, it is clear to me. If only, i've any doubts, may i post my questions ??
     
  13. May 20, 2013 #12
    I think what you are talking about here is static friction. When you have static friction, there is no relative sliding motion between the two bodies at their interface. The direction of the static friction force on the body is opposite to the direction that the body would tend to slide if such sliding were possible. In the case of a car going around a track, the car would tend to slide outward, so the static friction force is pointing inward. In the case of a block on an inclined plane, the block would tend to slide down the plane, so the static friction force on the block is up the plane.
     
  14. May 21, 2013 #13
    [REPLY] the instantaneous velocity of the car at a point in along the tangent. Ofcourse, friction is acting inwards i.e along the radius. Is it right to say that at an instant, friction is acting perpendicular to the direction of motion?
     
  15. May 21, 2013 #14

    Doc Al

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    Sure.
     
  16. May 21, 2013 #15

    sophiecentaur

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    I can only quote from Simon's first post, replying to your OP
    Imo, you should first sort out exactly what is causing circular motion (perfect ball on perfect string) and, where you see it operating in a complicated situation like a car, you will be able to identify the need for a centripetal force and be able to calculate it. This force is, of course, supplied by some of the friction from the tyres - as there's no 'string'. But a real car (unpowered would slow )down due to friction forces acting along a tangent, too. The overall (resultant) friction force direction will be somewhere in between the two and directed in a direction 'behind' the radius of motion. If the car is speeding up, the friction force will aim 'in front' of the radius. Only when the speed is constant will the net force from the tyres be radial.
     
  17. May 21, 2013 #16

    Doc Al

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    Note that the title of this thread specifies uniform circular motion.

    (Even then, of course, a real car must overcome air resistance so some component of tire resistance must be in the direction of motion.)
     
  18. May 21, 2013 #17

    sophiecentaur

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    Oh yes, of course. But my point was that the car is a complex system and so the other friction forces are acting all the time, even under uniform motion- when they just happen to balance out in the tangential direction. You need to include air friction as well as tyre friction, natch.
     
  19. May 21, 2013 #18
    I couldn't understand. Will you make it clear, how the resultant friction will be 'behind the radius' and 'in front of it' when the car slows down and speeds up respectively??
     
  20. May 22, 2013 #19

    sophiecentaur

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    There is a radial force keeping the car moving in a circle (this can only be due the friction from the tyres). There is a force slowing it down (or speeding it up), which acts tangentially. These are two vectors which can be added together(as with all vectors) to produce a resultant. This resultant will not point along the radius unless there is no speeding up or slowing down - therefore it will point either forwards or backwards (in front or behind) the radial direction.
    All this is very idealised, of course, because tyres are constantly slipping, even when the car is not actually under control and the driver may well need to be pointing the wheels in strange directions in order to keep on a curve - look up Slip Angle of Tyres, for more information.
    Does that make sense?
     
  21. May 29, 2013 #20
    now this is clear to me for sure. Thanks a lot! I've another question. It is about kinetic friction acting on a body. We all know, kinetic friction of a body is usually smaller than its static friction.
    (1) If we increase the speed of the body, what happens to its kinetic friction ? Does it increase, decrease, or remains the same ?
    (2) from what i know, the value of the 'coefficient of kinetic friction' generally doesn't change for the speeds up to less than 10m/s. What if we increase the speed? How does it affect the 'coefficient of kinetic friction' ?
     
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