 #1
Hamiltonian
 267
 181
 Homework Statement:

A block of mass 20kg is placed on another block of 10kg the coefficient of static friction between the two surfaces = 0.3
the surface of contact between the 10kg block and floor is smooth. A force of 30N acts on the 20kg block. comment on the motion of the system
 Relevant Equations:
 F = ma
the maximum value of friction between the surfaces of the blocks is ##60N##
the friction should be selfadjusting until this maximum value. Hence the force of friction in the given scenario should be = 30N but this is physically impossible as if this is the case the top block will be at rest and the bottom block will move.
hence I made the equations $$30  f = 20a $$ $$10a = f$$
from this I got ##f = 10N##
but I don't understand why we are supposed to form these two equations are we supposed to do this out of necessity as we cannot say friction will be selfadjusting in the given scenario as it will lead to a physically impossible situation?
I don't as such have an issue with the question rather I don't understand why the above equations yield a correct answer as here friction is notself adjusting.
the friction should be selfadjusting until this maximum value. Hence the force of friction in the given scenario should be = 30N but this is physically impossible as if this is the case the top block will be at rest and the bottom block will move.
hence I made the equations $$30  f = 20a $$ $$10a = f$$
from this I got ##f = 10N##
but I don't understand why we are supposed to form these two equations are we supposed to do this out of necessity as we cannot say friction will be selfadjusting in the given scenario as it will lead to a physically impossible situation?
I don't as such have an issue with the question rather I don't understand why the above equations yield a correct answer as here friction is notself adjusting.