What minimum value of ma will keep the system from starting to move?

Block on the table m (A) = m1,

block on the cord m2,

the coefficient of static friction is k1=0.4,

the coefficient of kinetic friction is k2 = 0.28

(a)

Block A:

T = F (fr) = k1 •N = k (s) • m1 •g,

Block B: T = m2•g.

k1 • m1 •g = m2•g,

m1 = m2/k (s) = m2/0.4.

(b)

Block A:

T = F (fr) = k2 •N = k2 • m1 •g,

Block B:

T = m2•g.

k2• m1 •g = m2•g,

m1 = m2/k2 = m2/0.28.