Consider a 7.0-kg red block pressed against a wall falling at -2.5 m/s. Given the following coefficients between the wall and the block--μk = 0.4 and μs = 0.7, determine the force of the hand that acts on the block. Use g = 9.79 m/s

Question

jamesj · Answer

The block is falling at a constant rate; that is not accelerating.  Hence the net force on the block--$ F_g $ for gravity and $ F_f $ for friction must cancel.

Now as $ F_f  = \mu N $, where $ N $ here is the normal force of the wall against the block, that must be the force supplied by the hand.  Hence you need to solve for the magnitude of $ N $ given that
$$ F_g + F_f = 0 $$