A particle which is moving
under gravity alone is moving in a straight line under constant acceleration,
so all the concepts applied to linear motion under constant acceleration
may equally be applied here.

On the ordinary level paper
the acceleration due to gravity is taken as 10 m/s^{2}, whereas
on the higher level the value for *g* is 9.8 m/s^{2}.

We adopt the same sign convention
here as in Cartesian geometry, namely "up" is positive, "down" is negative.

The equations of motion
are now:

**There are a number of
points worth noting:**

1. Stick to the sign convention

2. Take the point of projection
as the point where s = 0

This
means that if the particle goes below the point of projection its s value
is negative

3. At its highest point
the velocity of the particle is zero

4. The time the particle
is in the air may be determined by using

If the
particle lands at the same level at which it was projected then
**s
= 0**

If, however,
it lands* h* metres below its point of projection then,
**s
= - h**

5. If a particle is dropped from another, as the other moves, then the initial velocity of the particle is that of the one from which it was dropped.

*An example to illustrate
these points.*

**1992 Leaving Certificate
Higher Level.**

A baloon ascends vertically
at a uniform speed.

7.2 s after it leaves the
ground a particle is let fall from the balloon.

The particle takes 9 s to
reach the ground.

Calculate the height from
which the particle was dropped.

Particle:

Baloon:

Equate h's: