My teacher states that if we apply an external force just equal to the value of limiting friction, then the body will start moving. But if the external force is just equal to the maximum frictional force will not the net force on the body be zero? Can I conclude from this that the value of limiting friction can never be achieved?
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$\begingroup$ "Can I conclude from this that the value of limiting friction can never be achieved?" What do you mean.by "achieved"? $\endgroup$– Bob DCommented Feb 11 at 4:25
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$\begingroup$ I mean that the frictional force can never be equal to limiting friction? $\endgroup$– Madhav MittalCommented Feb 11 at 4:33
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2 Answers
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My teacher states that if we apply an external force just equal to the value of limiting friction, then the body will start moving
We generally say that when the applied force equals the limiting friction force motion is "impending”, meaning motion is imminent.
Can I conclude from this that the value of limiting friction can never be achieved?
No. The applied force can equal the limiting force but only briefly before motion occurs and static friction changes to kinetic friction, which is typically lower than static. That’s what we mean by “impending” motion.
A generic plot of friction resistance versus applied force can be found here:
http://hyperphysics.phy-astr.gsu.edu/hbase/frict2.html
Hope this helps.
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Technically, from $$ma = F_{inp} - \mu_sN $$ follows that for body to have $a \gt 0$, it must be true $F_{inp} \gt \mu_sN$. So you need to slightly overcome static friction for body to move.
Practically however, when you reach limiting friction $\mu_sN$ body ceases to experience surface traction, like it would be no friction at all on "ideal surface". Hence "any" slight perturbation of object still state will force it to move. It may be a little bit of shaking, blow of wind, imprecise input force (you are already overcoming static friction by a few $\mu N$, etc.)
Whatever shock to the system is, it suddenly jumps from static friction to the kinetic friction area as it is seen from this friction peak in chart :
answered Feb 11 at 8:35