# 2.3.1 Free Fall

Free fall refers to motion under the influence of gravity only. In other words, the mass experiences only the gravitational pull mg. Air resistance (and all the other forces) are absent or negligibly small.

It follows from Newton’s 2nd Law that all masses, heavy or light, free falls at the same rate.

$\displaystyle a=\frac{{{{F}_{{net}}}}}{m}=\frac{{mg}}{m}=g$

This is why g is called the acceleration of free fall. Its value is roughly $9.81\text{ m }{{\text{s}}^{{-2}}}$ on the surface of Earth.

If a mass is dropped from rest, the v-t and s-t graphs (↓ +ve sign convention) of the motion will be as follow:

• v-t graph is a straight line graph with gradient corresponding to 9.81 m s-2.
• s-t graph is a quadratic curve, with a steepening gradient corresponding to increasing speed.

Demonstrations

Free Fall (Apollo 15, Brian Cox and BRAINIAC)

Concept Test