1.

Block A weighs `4 N` and block weigh `8 N` The coefficient of kinetic friction is `0.25` for all surface find the force `F` to slide `B` at a constant speed when (a)`A` rest on `B` and moves with it (b) `A` is held at rest and (c )`A and B` are connected by a light cord passing over a smooth putting as shown in fig 7.31 (a - c) restively.

Answer» (a) When A moves with B the force opposing the motion is the only force of friction between B and S the horizontal and as velocity of system is constant ,
`F = f_(1) = muR_(1) = 0.25 ( 4 + 8) = 3N`
(b) When A is held stationary , the friction opposing the motion is between A and B , B and S . So ,
`F = muR_(1) + muR_(2) = 3 + 0.25(4)`
i.e., `" " F = 3 + 1 = 4N`
(c) In this situation for dynamic equilibrium of B
`F = muR_(1) + muR_(2) + T " "...........` (i)
while for the uniform motion of A .
` T = muR_(2) " " ..........` (ii)
Substituting T from Eqn. (ii) in (i) , we get
`F = muR_(1) + 2muR_(2) = 3 + 2 xx 1 = 5 N`


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