Question

i need help in Friction...!

Answer 1

i am in confusion about the direction of the friction...

Answer 2

in A the sphere is moving with a angular velocity of \(\omega\)
and in B F force is acted on the sphere....
draw the direction of the friction...

Answer 3

In A, if the sphere is rolling with constant angular velocity and there is no force acting elsewhere on the body, then friction will be zero.

Let us talk about part B. Choose some arbitrary direction of friction(backwards or forwards) and then find the angular acceleration and linear acceleration and apply the condition of rolling to find the value of friction force. If the assumed direction was wrong, your answer will contain minus sign.In this case ,friction will be forwards. So, try doing the question by assuming friction backwards and see that u get - sign.

Answer 4

Rule no1: If system is slipping, friction is opposed to relative motion wrt base.
Rule no2: If system is rolling, imagine you put lubricant or ice at point of contact, so that slipping would occur, then apply rule no1

Answer 5

I assume your base is fixed and horizontal.

In case A:
- if \(\omega\) is increasing, \(\vec f\) is pointing forwards;
- if \(\omega\) is decreasing, \(\vec f\) is pointing backwards;
- if \(\omega\) is constant, \(\vec f\) is zero.

In case B: If slipping was allowed, F would tend to make the solid rotate clockwise.
Hence slipping velocity ( = velocity of the point of the wheel in contact with the base) is to the rear, so friction is acting forward.

Answer 6

Let me elaborate Rule no1.
When I say "relative motion", I actually mean "slipping velocity", which is the velocity of the point of the system in contact with the base (minus velocity of the base if the base is moving too).

Answer 7

i do not understand this part...
if \(\omega\) is increasing , \(\vec{f}\) is pointing forward..
and if \(\omega\) is decreasing , \(\vec{f}\) is pointing backward...
?
if \(\omega\) is increasing, there must be a clockwise torque..(as \(\omega\) is clockwise)
so the friction would be pointing backward...!?
@Vincent-Lyon.Fr

Answer 8

It's a very good question, I don't understand why the friction force on the object rolling down a slope is pointing backwards instead of forwards, because it should acting against the motion of base particles.