A uniform cube of mass m & side a is rsting on a rough 45 degree inclined surface. The distance of the point of application of normal reaction measured from the lower edge of the cube is:a) zerob) a/3c) a/√2d) a/4

hi,

as normal rxn passes through centre & perpendicular to lower & upper faces & gap between centre & lower edge is

a/2^1/2

so (c) is correct. 

no!!! ans is (a)

hi, 

sorry for wrong ans.

now we will approach it as a case of toppling

MgsinΘ  is the component of weight which acts on centre at a distance a/2 from lower edge. so its torque will be

Τ1= MgsinΘ*a/2(.)

& normal  will nullify this effect so

T2 = MgcosΘ*r(×)

equating both

r=a/2 (from centre)

as normal passes through lower face

hence normal will pass through edge.

regards

anand