A Classical Approach of Newtonian Mechanics

4 NEWTON’S LAWS OF MOTION 4.9 Frames of reference

frictional force (which acts upwards) must exceed the downward acting weight,

mB g, of the block. Hence, we require

μ R > mB g,

or
g
a >.
μ

Applying Newton’s second law to the rightward acceleration a of both blocks
(remembering that the equal and opposite forces exerted between the blocks
cancel one another out), we obtain

F

a = ,

mA + mB

where mA is the mass of block A. It follows that

F >

(mA + mB) g

.

μ

Since mA = 75 kg, mB = 15 kg, and μ = 0.45, we have

F >

(75 + 15) × 9.81

0.45

= 1.962 × 103 N.