Question

when a circular body rolls down an incline, the instantaneous axis of rotation is the point of contact of the body and incline. So, the component of weight along the incline does produce some torque. so the friction has to act downwards. but we take it to be up the incline. why?

Answer 1

friction usually acts in the diection opposite to that of the net acceleration of body. In case of circular body rolling down an incline, the net acceleration is downwards. Hence friction opposes and acts upwards.

Plus, since the body is rotating about an axis passing through it's centre of mass, which is also the point at which the weight is acting, the weight does not produce a torque in this case.

Answer 2

but the instantaneous axis of rotation is the point of contact of the body and incline. so weight does produce torque.

Answer 3

Well, actually chand, the axis of rotation is the center of mass axis.That's what you see. A body can only rotate about an axis within itself, isnt it?
But to simplify rolling motion, we assume it to be a rotation about the point of contact.
During rolling, the body rotates about its center of mass axis and at the same time the center of mass moves in a straight line(or simply say the body translates. This makes things rather complicated. But since we know that the point of contact in pure rolling is at rest, we can consider the whole translation+rotation as a single rotation about the point of contact.
Do u get it?

Answer 4

right. got it.

Answer 5

So, u r from India?

Answer 6

yes

Answer 7

Yes, the friction is in the direction of 'up' the plane. Here's a worked example:
http://openstudy.com/study?login#/updates/4f42fe64e4b065f388dc742a