the work done by friction is negative

W=force of frictionxdistance cos180

W=(-ve) 

man walk due to friction hence angle btween friction and displacement is 0 not 180

TANWAR IS CORRECT

 when you walk we push the ground back but the friction tries to oppose the motion and acts in the forward direction. So the work done becomes positive.

Sorry Krishna Sir, But I disagree. There is no actual displacement of the point of contact while the friction acts on it, if walking is done without slipping. So how can work done be non-zero?

There has been a discussion on this topic before:

http://www.goiit.com/posts/list/mechanics-wat-is-the-work-done-by-friction-when-a-man-is-walking-972999.htm

walk and observe the various forces acting on ur leg.. u will find that ur pulling ur leg back and yet ur moving forward. Thats because the ground is pushing ur forward with the help of friction. and since the direction of friction is forward and ur also moving forward, the work done by friction is positive

@superman.. The foot not be moving but the bone attached to it is rotating in the forward direction. SO that rotational kinetic energy is the work done by friction

bt then also torque of friction abt the axis(pt. where friction is acting) of rotation is zero........isnt it??

Observe carefully:

.

U see the back foot is tilted.. the force of friction pushes the back foot bone forward.. this tilts the torso and gives the bone of the front foot some omega. which inturn becomes the rotational energy of the front foot.

FBD of the back foot. The component of force of friction "A" in the given figure pushes leg bone up thats why our foot tilts forward

This is my theory .. Its not from any book

Sorry the application point of force of friction is moving forward not backward. I animated it wrongly

@Mr. Nasty: Well done. Good work, it takes guts to answer differently after an expert has done it, but you are right.

Friction can never do positive work. Because even if the point of application moves, then too, the kinetic friction will oppose the motion. And hence work can't be positive.

In this particular question work done is zero.

@king: First of all, its not relevant to think of rotational motion here. Even if you do, torque is introduced by our muscles and not friction. If you jump, (neglect gravitational force), then will the work be done by your muscles or the normal reaction by the surface from which you jumped?

And even if we consider the torque to be the introduction by the force of friction, even so displacement is perpendicular to the applied force. Torque is applied here through the point of rotation, so it does no work in the rotatory sense of the problem too. Remember? Friction does no work in pure rolling.

Rotation was just a part of my argument. See theres a component of friction which is perpendicular to the heal of our foot. we can resolve the force of friction into two perpendicular directions, one of which is perpendicular to the heal of our foot. so when we tilt our back foot forward theres actually positive work done by that component. or u can say that that this component of friction is imparting some angular momementum to the foot. which is indeed work

Even though our muscles are tilting the foot, theres actually motion in the direction of that component of friction. so that component of friction does positive work.

See the case of two blocks tied with a string and passed over a pulley. Both tention and gravity do work. This is the same case.

And when we jump we are actually pushing the ground first. see it urself we first squirt and then we sort of fly.

@autobot

How can u say that work done by friction in pure rolling is zero.

See this example. the particle at instantaneous center is moving backward so friction is forward. and since COM is moving forward .. and friction force is acting on this cylinder/ sphere.. work done by friction is positive which is different from o as u said.

another proof that the direction of friction I assumed is correct is the fact that on a rough road a rolling cylinder tends to stop after some time cause friction generates a opposing torque, which is only possible if friction is acting the way I showed

Dear Student

In rolling, the motion that occurs due to friction is translatory. The torque is not applied by friction about the centre of mass, otherwise it would not have any translational movement. The torque acts about the point of contact, the angular displacement of the centre of mass is about the point of contact. That's the reason pure rolling actually occurs.

And even according to your diagram, if friction does positive work, the point of contact should move to the right, which would mean giving the body and anti clockwise, which does not happen. Friction can never do positive work, because the force on the point of application and the displacement of that point are always linearly opposite.

Since you have several doubts in the basic concepts here, try going through Fundamentals of Physics Part 1 by H. C. Verma. The phenomenon of pure rolling is well explained there.

On a rough road, pure rolling does not take place, and the direction of acting friction is to the left (in your diagram) which produces an opposing torque and hence stops the rolling.

And if you would pay heed to your own diagram, you will find that the torque is not produced about the center because of friction, because if it would have been so, the torque should have been anticlockwise, according to the direction of the frictional force, but it is clockwise.