[Physics] Free Fall with Air Resistance

airdragforcesfree fallnewtonian-mechanics

Can somebody help me understanding why when free falling the force on the vertical axis is F= -mg-kv, where k is the constant of air resistance and v the velocity?

Suppose the vertical axis is positive in the upwards direction. Then the acceleration is negative, so this is okay. The air resistance should be positive thou, cause it should oppose to the acceleration. Am I right or wrong?
Cause I don't understand how the air resistance and the acceleration could ever have the same sign. Indeed when opening a parachute, all we do is enhancing the air resistance and therefore balancing the two forces.

Best Answer

The force F is taken to be positive if it is upward and negative if it is downward. So -mg means that the gravitational force is downward. The term -kv means that, if the body is moving upward (positive v) the drag force is downward, and if the body is moving downward (negative v), the drag force is upward. This is completely consistent with our expectations.

Related Question