Freefall and

Acceleration Due to Gravity

If the only force acting on an object is the force of gravity (e.g., no air resistance, no propulsion), that object is said to be in freefall.

- The term is used whether the object is moving

upwards, downwards, or sideways.

- The condition of freefall ends whenever a new force

acts on the object (e.g., the object is caught or hits

the ground or a parachute opens or…)

Any object in freefall experiences a constant^* acceleration.

Since this acceleration is constant, it can be measured and an average value for a planet/star/moon assigned. On earth:

a = g = 9.8 m/s/s or 32 ft/s/s

This variable (and/or value) is NOT “gravity” or the “force of gravity” but the acceleration due to gravity.

*In fact, the value changes with elevation and location on the planet.

Acceleration Due to Gravity (continued)

Since the acceleration due to gravity is uniform and constant, any/all kinematics equations that involve acceleration are valid for objects in freefall.

Common math convention is that the vertical axis is the y-axis. Therefore, oftentimes, the kinematics equations for freefall are rewritten in terms of y rather than x. Also, you might see g replace a, for example:

y = v_o t + ½ gt²

Conventions vary, and one may change the convention as suits their need, but, the text’s general convention will be that upwardly directed vectors (displacements, velocities, accelerations, forces, etc) are assigned positive values and downwardly directed vectors are assigned negative values.