(This was actually an assignment)
Problem: A 400-kg bear grasping a vertical tree slides down at constant velocity. What is the friction force that acts on the bear? (Calculate it)
The first step is: understand the description of the problem/question.
2nd step: visualize it... Draw the situation... Show all the forces which are acting on the body considered ( the bear in this case )
3rd step: Since the forces are vectors, make a sign convention... In the diagram below, if the force is directed up it should have a negative (-) coefficient once involved in an equation, if downward, positive (+). (as indicated by the violet arrow).
4th step: Use the Newton's Second Law of Motion. Start the mathematical solution from the equation a = Fnet/m. (as shown)
Fnet = W - f , because f is directed up.
Why a = 0 ? Because, as stated the bear is sliding with constant velocity (which means as it slide its velocity does not vary). So the acceleration is zero.
Question #2: Is m really cancelled? (on Gravitational force on an object)
Answer: Yes. By a law of mathematics, it is cancelled. The specific, step-by-step derivation goes like this!
Time in Manila, Philippines 
