The three diagrams below show a block of mass m being pulled or pushed at constant acceleration ax along a table with a force P. The coefficient of kinetic friction is k. What is the friction force in each case? Give your answer in terms of the pushing force P, ax, m, and the angle .

Explanation:

1) F = ma

P - Fk = ma

Fk = P - ma

2) F = ma

Pcosθ - Fk = ma

Fk =  Pcosθ - ma

3) F = ma

Pcosθ - Fk = ma

Fk = Pcosθ - ma

These results are not very helpful.

If we are allowed to introduce gravity, more useful information comes out

1) Fk = kmg

2) Fk = k(mg + Fsinθ)

3) Fk = k(mg - Fsinθ)

it doesn't. it was imagined up in the middle ages.