# Physics with Calculus/Mechanics/Projectile Motion

## Projectile Motion

Using the equations we derived in the last section, we can now use them to model the motion of a projectile. A projectile is an object upon which the only force acting is gravity, which means that in all situations, the acceleration in the y direction, $a_{y}=-g$ . For simplicity, we will assume that the path of a projectile, also called its trajectory, will always be in the shape of a parabola, and that the effect of air resistance upon the projectile is negligible.

The Horizontal Motion

Since the only force acting upon the object is gravity, in the y direction, there is no acceleration in the x direction.

Let us assume that the projectile leaves the origin at time t = 0 and with speed vi. Then we have a vector vi that makes an angle of θi with the x-axis. Then, using a bit of trigonometry, we have the following:

$\cos \theta _{i}={\frac {v_{xi}}{v_{i}}}$

and

$\sin \theta _{i}={\frac {v_{yi}}{v_{i}}}$

Rearranging for the initial velocities, we get the initial x and y components of velocity to be

$v_{xi}=v_{i}\cos \theta _{i}$  and $\ v_{yi}=v_{i}\sin \theta _{i}$
$x=v\cos(\theta )t$

The Vertical Motion

There is a constant acceleration down, g which is the force of gravity. Accelecration is the instantaneous rate of change of velocity so:

${\frac {dv}{dt}}=a=g$

therefore we can integrate acceleration with respect to time to get velocity

$\int dv=\int gdt$
$v=gt$

Velocity is the instantaneous rate of change of displacement so:

${\frac {dd}{dt}}=v$

We can also integrate velocity with respect to time to get displacement

$\int dx=\int vdt$
$\int dx=\int gtdt$
$d={\frac {1}{2}}gt^{2}$