Question

At any given instant in time, a 4-kg rock that has been dropped from a high cliff experiences a force of air resistance of 15N.

What are the magnitude and direction of the acceleration of the rock? (don't forget gravitational force!)

Answer 1

m = mass of the rock  being dropped = 4 kg

g = acceleration due to gravity = 9.8 m/s²

W = weight of rock dropped in downward direction  = mg = 4 x 9.8 = 39.2 N

f = force due to air resistance in Upward direction = 15 N

a = acceleration of the rock in down direction

taking down as the positive direction of motion and Upward direction as negative

force equation for the motion of rock is given as

W - f = m a

39.2 - 15 = 4 a

a = 6.05 m/s²

direction : downward