71Naahh.. When you go to bed.
"Friction always acts upwards regardless of a spherical body rolling up or down a plane":
Is the above statement true?please explain.
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18 Answers
49Ashis: when u r pulling a body on a horizontal surface, it is not necessary friction is zero!!
0 friction is just a special case! [1]
30I hope we don't have to do such a thing in the exam (considering the time)! lol
71Everyone trying to learn rotation :
Close your eyes.. Imagine a sphere. Get it rolled anywhere you want. No reason yourself where the friction will act and where will it not.
After 30 mins, you'll certainly clear your concepts that you haven't been able for long.
49no friction acts upwards..
It gives the retarding torque to the body! :)
11consider this situation
the ball is rolling from A to B.
wont the frictional force while negotiating the upward slope act downwards??
30I think it is static. Because the question of kinetic friction arises only when there is relative motion.
71@Ashish
" frictional force " is zero during pure rolling.
Q: It is what type of frictional force that is zero :
a) Static
B) Kinetic
C) Both
49not necessarily:
In this case it isn't:
(the sphere is accelerating up the incline!)
Try to reason out why.. in case at doubt i will help out ! [1]
30Yeah. I was just giving an example where frictional force is zero during pure rolling.
30Bhaiya are you talking of the original problem of this thread?
49Ashish: u are a bit wrong.. friction does act but in this case it is in downward direction...the only force providing the required force about the centre of mass! [1]
30@saikat_092 : Think of a situation where the body is undergoing translational and rotational motion with respect to the ground with VCM=ωr such that at every instant the velocity of the point of contact of the body with the ground is zero. Frictional force will act only if there is relative motion between the ground and the body, which in this case is not happening. Hence, no friction force acts and body undergoes pure rolling! [1]
1see...in above case...
F lag rahi hai => velocity of COM will increase.
but there ain't any torque abt COM...so omega can't increase//...
so V=RW wont hold...
thus friction must act..to increase W..
ie. if there is motion with F> or < mgsinQ....friction must act..
in case F=mgsinQ.... friction won't act for 'pure rolling condition'....
1Tell me something,is friction always necessary for this body to "roll" up or down the plane?if not,can you describe a situation,in absence of friction,where the body rolls?
262@ subhomoy- read the question properly. it is written that while a body is " rolling" up or down the plane and therefore the only force is gravitational hence a body cannot accelerate upwards!