Block A as shown in figure weight `1.40 N` and block B weight `4.20 N` The coefficient of kinetic friction between all surface is `0.30` Find the magnitude of the horizontal force (in N) necessary to drag block B to the left at constant speed if A and B are connected by a light , flexible cord passing around a fixed , frictionless pulled.

Answer

` = 2.52 N`

Explanation

The friction force on block A is `mu_(k) w_(A) = (0.30)(1.40 N) = 0.420N` This is the magnitude of the friction force that block A exerts on block B as well as the tension in the string because speed is constant . The force `F` must then have magnitude
`F = mu_(k)(w_(B) + w_(A)) + mu_(k) w_(A) + T`
`= mu_(k)(w_(B) + w_(A))`
`= (0.30)(4.20N + 3(1.40 N)) = 2.52 N`
Note that the normal force exerted on block B by the table is the sum of the wedge of the blocks.