The projectile motion equations from your text (two-dimensional constant-acceleration kinematics equations with a_x = 0 and a_y = -9.8 m/s²) are valid when an object is subject to the influence of gravity alone. The usual suspect that makes the equations invalid is air resistance (drag). Air resistance is significant in comparison to gravity if any of the following are true:

1) Small object (ex: dust)

2) Low density object (ex: balloon)

3) Fast object (ex: bullet)

4) Broad cross sectional area object (ex: parachute).

An Olympic athlete runs and then jumps as far as they can. Categorize their motion from just after they leave the ground to just before they land in the sand pit.

A) Air resistance is not significant. The projectile motion equations are valid.

B) Air resistance is significant. The projectile motion equations are not valid.

Solution