# Projectile motion

Parabolic water trajectory
Initial velocity of parabolic throwing
Components of initial velocity of parabolic throwing

Projectile motion is a form of motion in which an object or particle (called a projectile) is thrown near the earth's surface, and it moves along a curved path under the action of gravity only. The implication here is that air resistance is negligible, or in any case is being neglected in all of these equations. The only force of significance that acts on the object is gravity, which acts downward to cause a downward acceleration. Because of the object's inertia, no external horizontal force is needed to maintain the horizontal motion.

## The initial velocity

Let the projectile be launched with an initial velocity ${\displaystyle \mathbf {v} _{0}}$. which can be expressed as the sum of horizontal and vertical components as follows:

${\displaystyle \mathbf {v} _{0}=v_{0x}\mathbf {i} +v_{0y}\mathbf {j} }$.

The components ${\displaystyle v_{0x}}$ and ${\displaystyle v_{0y}}$ can be found if the initial launch angle, ${\displaystyle \theta }$, is known:

${\displaystyle v_{0x}=v_{0}\cos \theta }$,
${\displaystyle v_{0y}=v_{0}\sin \theta }$.

See also the section Parabolic equation for information on finding the initial velocity.

## Kinematic quantities of projectile motion

In projectile motion, the horizontal motion and the vertical motion are independent of each other; that is, neither motion affects the other. This is the principle of compound motion established by Galileo in 1638.[1]

### Acceleration

Since there is only acceleration in the vertical direction, the velocity in the horizontal direction is constant, being equal to ${\displaystyle \mathbf {v} _{0}\cos \theta }$. The vertical motion of the projectile is the motion of a particle during its free fall. Here the acceleration is constant, being equal to ${\displaystyle g}$.[2] The components of the acceleration are:

${\displaystyle a_{x}=0}$,
${\displaystyle a_{y}=-g}$.

### Velocity

The horizontal component of the velocity of the object remains unchanged throughout the motion. The downward vertical component of the velocity increases linearly, because the acceleration due to gravity is constant. The accelerations in the ${\displaystyle x}$ and ${\displaystyle y}$ directions can be integrated to solve for the components of velocity at any time ${\displaystyle t}$, as follows:

${\displaystyle v_{x}=v_{0}\cos(\theta )}$,
${\displaystyle v_{y}=v_{0}\sin(\theta )-gt}$.

The magnitude of the velocity (under the Pythagorean theorem, also known as the triangle law):

${\displaystyle v={\sqrt {v_{x}^{2}+v_{y}^{2}\ }}}$.

### Displacement

Displacement and coordinates of parabolic throwing

At any time ${\displaystyle t}$, the projectile's horizontal and vertical displacement are:

${\displaystyle x=v_{0}t\cos(\theta )}$,
${\displaystyle y=v_{0}t\sin(\theta )-{\frac {1}{2}}gt^{2}}$.

The magnitude of the displacement is:

${\displaystyle \Delta r={\sqrt {x^{2}+y^{2}\ }}}$.

## Parabolic equation

Consider the equations,

${\displaystyle x=v_{0}t\cos(\theta )}$,
${\displaystyle y=v_{0}t\sin(\theta )-{\frac {1}{2}}gt^{2}}$.

If t is eliminated between these two equations the following equation is obtained:

Since ${\displaystyle g}$, ${\displaystyle \theta }$, and ${\displaystyle \mathbf {v} _{0}}$ are constants, the above equation is of the form

${\displaystyle y=ax+bx^{2}}$,

in which ${\displaystyle a}$ and ${\displaystyle b}$ are constants. This is the equation of a parabola, so the path is parabolic. The axis of the parabola is vertical.

If the projectile's position (x,y) and launch angle (θ or α) are known, the initial velocity can be found solving for ${\displaystyle v_{0}}$ in the aforementioned parabolic equation:

${\displaystyle v_{0}={\sqrt {{x^{2}g} \over {x\sin 2\theta -2y\cos ^{2}\theta }}}}$.

## Time of flight or total time of the whole journey

The total time ${\displaystyle t}$ for which the projectile remains in the air is called the time of flight.

${\displaystyle y=v_{0}t\sin(\theta )-{\frac {1}{2}}gt^{2}}$

After the flight, the projectile returns to the horizontal axis, so y=0

${\displaystyle 0=v_{0}t\sin(\theta )-{\frac {1}{2}}gt^{2}}$
${\displaystyle v_{0}t\sin(\theta )={\frac {1}{2}}gt^{2}}$
${\displaystyle v_{0}\sin(\theta )={\frac {1}{2}}gt}$
${\displaystyle t={\frac {2v_{0}\sin(\theta )}{g}}}$

Note that we have neglected air resistance on the projectile.

## Maximum height of projectile

Maximum height of projectile

The greatest height that the object will reach is known as the peak of the object's motion. The increase in height will last until ${\displaystyle v_{y}=0}$, that is,

${\displaystyle 0=v_{0}\sin(\theta )-gt_{h}}$.

Time to reach the maximum height:

${\displaystyle t_{h}={\frac {v_{0}\sin(\theta )}{g}}}$.

From the vertical displacement of the maximum height of projectile:

${\displaystyle h=v_{0}t_{h}\sin(\theta )-{\frac {1}{2}}gt_{h}^{2}}$
${\displaystyle h={\frac {v_{0}^{2}\sin ^{2}(\theta )}{2g}}}$ .

## Relation between horizontal range and maximum height

The relation between the range ${\displaystyle R}$ on the horizontal plane and the maximum height ${\displaystyle h}$ reached at ${\displaystyle {\frac {t_{d}}{2}}}$ is:

${\displaystyle h={\frac {R\tan \theta }{4}}}$

### Proof

${\displaystyle h={\frac {v_{0}^{2}\sin ^{2}\theta }{2g}}}$

${\displaystyle R={\frac {v_{0}^{2}\sin 2\theta }{g}}}$
${\displaystyle {\frac {h}{R}}={\frac {v_{0}^{2}\sin ^{2}\theta }{2g}}}$ × ${\displaystyle {\frac {g}{v_{0}^{2}\sin 2\theta }}}$
${\displaystyle {\frac {h}{R}}={\frac {\sin ^{2}\theta }{4\sin \theta \cos \theta }}}$

${\displaystyle h={\frac {R\tan \theta }{4}}}$

## Maximum distance of projectile

Main article: Range of a projectile
The maximum distance of projectile

It is important to note that the range and the maximum height of the projectile does not depend upon its mass. Hence range and maximum height are equal for all bodies that are thrown with the same velocity and direction.

The horizontal range d of the projectile is the horizontal distance it has travelled when it returns to its initial height (y = 0).

${\displaystyle 0=v_{0}t_{d}\sin(\theta )-{\frac {1}{2}}gt_{d}^{2}}$.

Time to reach ground:

${\displaystyle t_{d}={\frac {2v_{0}\sin(\theta )}{g}}}$.

From the horizontal displacement the maximum distance of projectile:

${\displaystyle d=v_{0}t_{d}\cos(\theta )}$,

so[3]

${\displaystyle d={\frac {v_{0}^{2}}{g}}\sin(2\theta )}$.

Note that ${\displaystyle d}$ has its maximum value when

${\displaystyle \sin 2\theta =1}$,

which necessarily corresponds to

${\displaystyle 2\theta =90^{\circ }}$,

or

${\displaystyle \theta =45^{\circ }}$.

## Application of the work energy theorem

According to the work-energy theorem the vertical component of velocity is:

${\displaystyle v_{y}^{2}=(v_{0}\sin \theta )^{2}-2gy}$.

## Projectile motion in art

• The sixth panel of Hwaseonghaenghaengdo Byeongpun (화성행행도 병풍) describes King Shooting Arrows at Deukjung Pavilion, 1795-02-14. According to palace records, Lady Hyegyeong, the King's mother, was so pleased to be presented with this 8-panel screen of such magnificent scale and stunning precision that she rewarded each of the seven artists who participated in its production. The artists were Choe Deuk-hyeon, Kim Deuk-sin, Yi Myeong-gyu, Jang Han-jong (1768 - 1815), Yun Seok-keun, Heo Sik (1762 - ?) and Yi In-mun.
Arrows
득중정어사
화성행행도 병풍
Hwaseonghaenghaengdo Byeongpun

## References

• Budó Ágoston: Kísérleti fizika I.,Budapest, Tankönyvkiadó, 1986. ISBN 963 17 8772 9 (Hungarian)
• Ifj. Zátonyi Sándor: Fizika 9.,Budapest, Nemzeti Tankönyvkiadó, 2009. ISBN 978-963-19-6082-2 (Hungarian)
• Hack Frigyes: Négyjegyű függvénytáblázatok, összefüggések és adatok, Budapest, Nemzeti Tankönyvkiadó, 2004. ISBN 963-19-3506-X (Hungarian)

## Notes

1. ^ Galileo Galilei, Two New Sciences, Leiden, 1638, p.249
2. ^ The ${\displaystyle g}$ is the acceleration due to gravity. (${\displaystyle 9.81m/s^{2}}$ near the surface of the Earth).
3. ^ ${\displaystyle 2\cdot \sin(\alpha )\cdot \cos(\alpha )=\sin(2\alpha )}$
• The equations in this article are approximations that hold when the projectile's velocity is small relative to the escape velocity. Faster-moving projectiles will follow a sub-orbital trajectory or go into orbit.