# 6.1: Projectile Motion

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The simplest case of two-dimensional motion occurs when a particle experiences a force only in one direction. The prime example of this case is the motion of a projectile in Earth’s (or any other planet’s) gravitational field as locally described by Galilean gravity (Equation 2.2.2): $$\boldsymbol{F}=m \boldsymbol{g}$$. Once a projectile has been fired with a certain initial velocity $$\boldsymbol{v}_{0}$$, we can find its trajectory by solving the equation of motion that follows from Newton’s second law: $$m \boldsymbol{g}=m \ddot{\boldsymbol{r}}$$. We can decompose $$\boldsymbol{r}$$ and $$\boldsymbol{v}_{0}$$ in horizontal (x) and vertical (z) components; each of them has its own one-dimensional equation of motion, which we already solved in Section 2.3. The horizontal component experiences no force and thus executes a simple linear motion with uniform velocity $$v_{0} \cos \theta_{0}$$, where $$\theta_{0}=\arccos \left(\boldsymbol{v}_{0} \cdot \hat{\boldsymbol{x}}\right) / v_{0}$$ is the angle with the horizontal under which the projectile was fired and $$v_{0}=\left|\boldsymbol{v}_{0}\right|$$ the initial speed. Likewise, because the acceleration due to gravitation is constant, our projectile will execute a uniformly accelerated motion in the vertical direction with initial velocity $$v_{0} \sin \theta_{0}$$. If the projectile’s initial position is $$\left(x_{0}, z_{0}\right)$$, its motion is thus described by:

$\boldsymbol{r}(t)=\left(\begin{array}{c}{x(t)} \\ {z(t)}\end{array}\right)=\left(\begin{array}{c}{x_{0}} \\ {z_{0}}\end{array}\right)+v_{0}\left(\begin{array}{c}{\cos \theta_{0}} \\ {\sin \theta_{0}}\end{array}\right) t-\left(\begin{array}{c}{0} \\ {g}\end{array}\right) \frac{1}{2} t^{2}$

This page titled 6.1: Projectile Motion is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Timon Idema (TU Delft Open) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.