Can anyone teach me how to find the value of friction and direction as well in rollling?

Question

dls · Answer

@Mashy @RnR ^_^
as usual :D

anonymous · Answer

ok.. in rolling there are two cases

best example is one i give in class

if you are sitting on a bicycle.. and trying to pedal.. in taht case the cycle moves forward why?? 

imagine if there was no friction.. the tyres would just keep spinning.. but frction opposes that.. |dw:1363875261341:dw|

so friction acts in the direction shown

now imagine.. instead of sitting on the bike and peddling.. standing and pulling the bike as you walk.. in this case if there was friction.. then the bicycle would just skid.. so frction avoids that.. 
|dw:1363875369329:dw|

so in the two cases.. the frictions acts in opposite directions to provide rolling!