the 7kg brick described as (horizontal force of 42N, frictional force of 18 N)is now dropped off a cliff. At one point during the drop the friction due to air resistance has a strength of 14N. what is the acceleration of the brick at this time
force down = m g = 7 * 9.81 = 68.7 N
force up = 14
net force down = 68.7 - 14 = 54.7 N
so
m a = 54.7
a = 54.7/7 = 7.81 m/s^2