Question

How much force is required to come in motion for solid weights 20 kg is placed on a
solid surface if the coefficient of friction is 0.25?
a) 4.8 kg
b) 4.6 kg
c) 4.4 kg
d) 4.2 kg​

Answer 1

Explanation:

At the start the 100N force is just enough to overcome static friction so we can write:

Force=Static Friction

F=μsN

where μs is the coefficient of static friction and N= Normal Reaction that in an horizontal case such this will be equal to the weight of the block, so N=mg.

We get:

F=μs⋅mg

in numbers:

100=μs⋅20⋅9.8

μs=10020⋅9.8=0.51

When the movement starts, kinetic friction kicks in and we have that to have uniform motion we need acceleration equal to zero (constant velocity).

We use Newton's Second Law: Σ→F=m→a

or in our case:

Force−Kinetic Friction=mass⋅acceleration

or

F−μkN=0 because acceleration has to be zero.

F−μk⋅mg=0

in numbers:

F−0.4⋅20⋅9.8=0

F=78.4N