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22 Dec 2007 14:40:33 IST
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tka
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Joined: 16 Dec 2007
Posts: 432
22 Dec 2007 15:18:28 IST
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in both cases, i assume two things:
1.) no brakes are being applied
2.)the tyres roll without slipping
a popular misconception is that when rolling, there is a tendency for friction to act in the same direction as the tendency for motion of the center of mass. at a given instant during rolling , the point at instantaneous contact with the ground is at rest**. so friction acts in the opposite direction too. this should answer both your questions, ie.
that when there is no slipping, friction acts along east for both.
** how? well, consider the diagram.
so now you agree that there is no tendency for motion at the instantaneous point of contact to move backwards. this means, FRICTION ACTS BACKWARDS AND NOT FORWARDS
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22 Dec 2007 15:25:17 IST
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See this:
a)Back wheel-towards west
I think the same for the front wheel also..
b)For constant velocity,external forces acting should be zero,
So friction is 0,hence no direction.
22 Dec 2007 15:29:07 IST
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how does friction become 0 if external forces are 0? remember you accelerate the cycle too, and it is just balances w/ friction. this does NOT mean friction is 0
and i dont agree with your answer that both wheels toward west. It's kinda obvious that in both wheels friction acts towards same direction but i dont think towards west.
22 Dec 2007 15:44:16 IST
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Assuming pure rolling,(like you did too),at any instant just 1 point is in contact with the ground...and that point moves backward at that instant..so,friction is in opp. direction,ie westwards..
About uniform velocity,even I had the same doubt..that maybe forces are in equilibrium-as is the case with a drop falling with terminal velocity...
But a similar question in HCV said it should be zero....I'm waiting for someone to clarify it.
22 Dec 2007 15:48:28 IST
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there's the catch. just one point that glares in ur thesis
Assuming pure rolling,(like you did too),at any instant just 1 point is in contact with the ground...and that point moves backward at that instant..so,friction is in opp. direction,ie westwards..
have a look at my diagram. probably it will convince you that even though the point moves backwards, friction acts in the SAME direction as it would if there was no rotation
NOTE: I used to think the same way as you did. but consider this case where a ball rolls down an incline, if the friction acts forwards , then an instant after motion it will tend to act backwards (since tendency of motion is now forwards) , which means it should resist motion as it usually does. think over this
23 Dec 2007 03:30:24 IST
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The force of friction acts tangentially to the surface of the wheel at the point of its contact with the road (point A & B in the diagram above).
But we here are talking about the direction of friction. It doesnt matter whether the body is accelerating or not. In both the cases (a and b), the direction of the friction will be in the forward direction for both the wheels. Just in case two, the frictional force will be balanced by the force applied. (uniform acceleration)
Reason??? Given below
23 Dec 2007 03:44:41 IST
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Remember, that friction is the force which opposes the force being applied i.e. opposes the motion of the body .. .. .. .. (i)
Look at the diagram above.
Try to find the direction of applied force at each point of the wheel. You will find that the direction of applied force is backwards at the lowermost point of the wheel.
Therefore,
From (i), we have
The direction of the friction in the forward direction. (opposite to F)
Note that we are considering the forces acting on the circumference of the wheel.... This is because we want the direction of friction, at the point of contact of the wheel with the ground.
23 Dec 2007 04:00:02 IST
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Understand and analyze this carefully,
Experiment: To know the direction of force (F) being applied!!
Try to rotate a wheel with your hand in forward direction.
Where will you apply the force???
At the topmost point we apply it in forward direction and
At the lowermost point we apply it in backward direction (ie we pull the wheel backwards from below)
so that the wheel moves forward.
look at the diagrams 3 and 4 for more understanding
So then, what will be the direction of friction at the bottom most point?
Now its very simple, isnt it.
23 Dec 2007 04:14:25 IST
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Case I (a)
The body is accelerating,
We have resultant of all the applied forces F > f
Here the resultant of all the applied forces is in forward direction, but we are concerned only with the direction of the force at the lowermost point to get the direction of frictional force.
Case II (b)
As the body has zero acceleration,
We have resultant of all the applied forces F = f
Finally salute if found useful !!!
.
.
23 Dec 2007 04:15:21 IST
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Did you like the diagrams???
I think, I havent wasted my time in writing something completely crap. This info is not given in any source. Its my own imagination, and according to me everything I have written is right. Please dont boink if you feel the stuff rubbish. Nudge me if you have some doubts.
With all my efforts,
Indzee
21 Feb 2008 01:39:58 IST
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IN THE CASE OF BICYCLE,
WHEN ONE PEDALS, THE PEDALLING COMMUNICATES THE FORCE(external) ON REAR WHEEL ,WHICH MAKES THE FRONT WHEEL TO MOVE. SO, THE REAR WHEEL'S POINT OF CONTACT GETS A TENDENCY TO MOVE BACKWARD, friction would like to oppose it and be to its FWD dirn.
but as for the case of front wheel it just moves because the REAR wheel moves and the system must move forward.
.
Hence during pedalling, the force of friction acts,
BACKWARD DIRECTION ON THE FRONT WHEEL BUT FORWARD DIRECTION ON THE REAR WHEEL.. (case when cycle accelerates)
WHEN ONE DOESNOT PEDALS AT ALL ,
THE BODY GETS TO MOVE JUST DUE TO INERTIA OF MOTION
THE FORCE OF FRICTION ACTS IN BACKWARD DIRECTION ON BOTH THE WHEELS...
(case when cycle doesn't acclt.)
21 Feb 2008 15:30:47 IST
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but indzee when one is moving on a bicycle , he gives torque at the centre of the backward wheel to rotate in east direction at bottom point therefore the friction force should act in west
IN FRONT WHEEL THE COM IS MOVING IN WEST AND AT BOTTOM OF WHEEL THE FRICTION SHOULD ACT IN EAST DIRECTION
PLEASE PONDER UPON IT & TELL ME IF I AM RIGHT OR WRONG
BY THE WAY WHAT IS THE CORRECT ANS
IN FRONT WHEEL THE COM IS MOVING IN WEST AND AT BOTTOM OF WHEEL THE FRICTION SHOULD ACT IN EAST DIRECTION
PLEASE PONDER UPON IT & TELL ME IF I AM RIGHT OR WRONG
BY THE WAY WHAT IS THE CORRECT ANS
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