Reading the situation first

Friction problems have no formula to memorise. They have a question to answer first, and once it is answered the rest is arithmetic you already know.

Always ask this first

Is the object moving?

Everything else follows from the answer, because it decides which of the three frictions you are dealing with.

The three branches

The problem says	The friction is	Then
Not moving	Static — equal to the applied force	The forces balance; nothing to work out
Just about to move	Limiting — the maximum	The applied force equals the limiting friction
Sliding	Dynamic — given in the problem	Find the unbalanced force, then use F = m a

The two relations you need

While it slides

unbalanced force = applied force − friction

And then chapter 4 takes over: that unbalanced force is the F in F = m a.

Pressing on the surface

R = weight (on a horizontal surface)

Stack something on top and R rises by that much, and so does the limiting friction.

One block, six examples

The next two pages work through six problems on the same block from activity 01: weight 60 N, limiting friction 18 N, dynamic friction 15 N.