IIT JEE , AIEEE Forum - Physics , Mechanics

atri.shivam

what is the direction of friction on the front and back tyres of a cycle going west when
a.cycle is accelerating
b.velocity of cycle is constant
why?

tka

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

learner

See this:

http://www.goiit.com/posts/list/mechanics-direction-of-friction-small-challenge-35760.htm

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.

tka

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.

learner

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.

tka

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

atri.shivam

you are both wrong
the dirction of friction is backward on front wheel and forward and front wheel
this ques came in olympiad this year and also is in d c pandey
of course book maybe wrong
i think that friction acts forward in both wheels

indzee

The bicycle

indzee

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

indzee

Diagram 2

indzee

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.

indzee

Experiment diagrams

indzee

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.

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