A firefighter of mass 120 kg slides down a vertical pole with an acceleration of 4 m/s². What is the friction force that acts on the firefighter?
In this case, the friction force is the force that is acting against gravity.
Use F = M * a
If g = 9.8 m/s² and the firefighter's acceleration is 4 m/s², then (9.8 - 4) m/s² is the deceleration supplied by the firefighter's hands (or legs, whatever part of the body he/she is using to decelerate). So the force is F = M * (9.8 - 4) m/s², where M is the firefighter's mass.