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# Physics/Direction of friction force

Question
QUESTION: Sir please explain what is the direction of friction in rolling motion? I read resnick halliday and it said that friction acts in the direction opposite to the direction in which the body slips but how do I know in which direction will the body slip? Also often if i accelerate a body forward friction is backwards but in some cases even if I apply a force in the forward direction friction acts backward.Same is the case in rolling of a body on an inclined plane.Why is it so? Please help me out sir.

ANSWER: The concept of friction force, including its direction, is simple, even if the math can become complicated. If you get the concept correct, then the math should follow.

Remember that every force, including friction, has the effect of changing the motion of an object. The main concept you have to understand about friction is that it ALWAYS slows down an object. No matter what direction the object may be moving, the friction force is ALWAYS in the direction OPPOSITE to that motion. So, if you move an object forward, friction will be in the backward direction; if you move an object backward, friction will be in the forward direction. So just look at the system and ask ONE question: "In which direction is the object moving?" Once you answer that question, then you KNOW which direction is the friction force is in.

In the sketch just below, the 'X' is an object moving to the right (as viewed on the screen).

Time 1  X
Time 2     X
Time 3        X
Time 4          X

If we happen to define the x-axis such that motion to the right is positive, then the object X is moving in a positive direction. This means the friction force MUST be in the negative direction. This friction is always working to slow down the motion. Once you clear that concept, then the math should come a little more easily.

> how do I know in which direction will the body slip?

Once again, don't worry about the math AT FIRST. Just ask yourself this question: which way will an object move *IF* there is no friction?
For instance, if a box is on an inclined plane, and "forward" (ie, in the positive-x direction) is defined as down the plane, which way would the object slip with no friction? Obviously, down the plane, and thus forward. Note that, if you happen to define down the plane as "backward" (ie, in the negative-x direction), then the box will slip backward.
But, again, just look at the system and think of which way the object would move. THEN do the math.

---------- FOLLOW-UP ----------

QUESTION: But sir I think there are some other things that we have to look for in case of rolling motion because the object not only undergoes translatory motion but also rotates about its axis.In some cases even if the acceleration is in forward direction the friction too acts in forward diection but in other cases it does not follow the same notion.So??

Could you refer me to a URL that shows what situation you are discussing?

If an object is SLIDING down a plane, then friction is (as I noted) in the direction opposite to the motion.

If the object is rotating as it goes down the plane, then we (pretty much) neglect friction, simply because the object would rotate down the plane no matter how much would be the friction between the rotating object and the plane. A rotating object would reach the bottom in about the same amount of time whether the plane was smooth metal or sandpaper.

http://www.damtp.cam.ac.uk/user/reh10/lectures/ia-dyn-handout16.pdf
Discusses the situation in more rigorous detail.

Why do we neglect friction for a rotating object? Different game, different rules!

---------- FOLLOW-UP ----------

QUESTION: Sir I mean that in resnik halliday there was a topic in which they explained the roll of friction in rolling motion.They gave an example that suppose a wheel is made to accelerate in the right direction then the force of friction acts in the right direction as well.But friction always opposes the relative motion or as you said slows down an object as you said then the direction of friction should be to left.Then why is it acting in the direction of force applied?

I regret that I STILL do not fully understand your question, because you are quoting from a text book that I do not have in front of me. I suggest you discuss this with your instructor, who can look at the book and discern the exact nature of the problem.

Physics

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