IIT JEE , AIEEE Forum - Physics , Mechanics

piyushshukla

in pure rolling velocity of point of contact remains zero i.e. there is no relative sipping then how does friction comes into play

ramyani

1. velocity   of point of contact remains zero ---- this concept is better used when treating the problem with Instantaneous center ( IC ).

The combined effect of translation of the CM and rotation abt an axis thro the CM is equivalent to a pure rotation with the same angular speed abt an axis passing thro a point of zero velocity.Such an axis is called instantaneous axis of rotation ( IAOR ).

The axis is always perpendicular to the plane used to represent the motion. The intersection of the axis with the plane defines the location of the instantaneous centre ( IC) of zero velocity.

For example, consider a ring or wheel which rolls without slipping. In this case, the point of contact with the ground has zero velocity. Hence this point is the IC for the ring or wheel.

If it is imagined that the wheel is momentarily pinned at this point, the velocity of any point of the wheel can ve found out by using

                          V = R W.

Here R is not radius, but distance of the point from IC.

Thus the KE of a body can be assumed pure rotational abt IAOR.

                  KE = ( 1 / 2) I_IAORW²

rotation + translation = pure rotation abt IAOR through IC

KE = ( 1 / 2) I_CM W² + ( 1 / 2) mV² = ( 1 / 2) I_IAORW²

2. Now let us just remember that everybody wants to attain the " ideal pure rolling " a = r

condn. (

= angular accln ; a= linear accln )

When actually pure rolling, takes place ( a = r

) , then friction is taken as zero. There is no need for friction.

If a < r

, then to support the linear motion, friction acts in the forward dirn.

krishna.gopal

Try pushing a heavy stone. You won't be able to move it and relative motion IS ABSENT. Still friction is there which is balancing your force.
Relative motion is not necessry for friction. Relative motion means that there is friction. But its absence does not mean that there is no friction

piyushshukla

i think ur answer is wrong

rajat.khanduja

In pure rolling, friction acts to force pure rolling. In case the body is already in pure rolling, then no friction acts.

If another torque is being applied on the body, friction might act in order to force pure rolling

anchal_1991

yup rajat is rite theres no friction in pure rolling motion

gauravstud

then how do we use friction in problems explain a bit

papu071291

what is the role of friction in pure rolling

gauravstud

hi experts reply with a correct answer.

ramyani

1. In pure rolling motion friction is static in nature and work done by friction is zero.

2.In general , friction is responsible for pure rolling motion.

3. In Impure rolling motion friction is kinetic in nature and direction of friction is decided by the relative velocity of the point of contact with the platform.

4.It is a misconception that V = Rw for pure rolling. It is the case when the platform is stationary.

bhuwanaroracorroded

d friction will not act once d pure rollin condition is achieved

shashankparewar

frictin only prevents the slipping of the ball to continue the rolling the motion.if there is no friction there will be slipping and rolling cannnot occur.when there is no rolling the friction is kinetic and helps too attain the state of rolling.once pure rolling starts there will be no friction at all because there is no rtelative motion.

ashwin123

Arre, then how do u explain a rolling body coming to rest?It is because of another type of friction (rolling friction ) that acts on the body at the same time.(See HCV)This friciton acts on the body due to the hardness of the surface and not the roughness of the surface.Actually when a body is on the ground na, a deformation is created in the surface (see fig)(This is coz no body is perfectly hard). . Coz of that a normal force acts on the body and uska parallel component, is the rolling friction which opposes the motion of the COM .

But as far as sliding friction is concerned it only tends to bring the relative motion of the p.o.c to a zero. AT PURE ROLLING , SLIDING FRICTION IS ZERO,BUT THE FRICTION IS NOT ZERO. (kyunki sliding friction ka kam khatam.).

ashwin123

Imagine that the COM is moving to the right.

Ask & Discuss Questions with Community & Experts