Projectile Motion

What is a Projectile?

Projectile is any object thrown into the space upon which the only acting force is the gravity. In other words, the primary force acting on a projectile is gravity. This doesn’t necessarily mean that the other forces do not act on it, just that their effect is minimal compared to gravity. The path followed by a projectile is known as trajectory. A baseball batted or thrown and the instant the bulllet exits the barrel of a gun are all examples of projectile.

What is Projectile Motion?

When a particle is thrown obliquely near the earth’s surface, it moves along a curved path under constant acceleration that is directed towards the center of the earth (we assume that the particle remains close to the surface of the earth). The path of such particle is called a projectile and the motion is called as projectile motion. Air resistance to the motion of the body is to be assumed absent in projectile motion.

In a Projectile Motion, there are two simultaneous independent rectilinear motions:

  1. Along x-axis: uniform velocity, responsible for the horizontal (forward) motion of the particle.
  2. Along y-axis: uniform acceleration, responsible for the vertical (downwards) motion of the particle.

Accelerations in the horizontal & vertical direction of a particle in projectile motion: When a particle is projected in the air with some speed, the only force acting on it during its time in the air is the acceleration due to gravity (g). This acceleration acts vertically downward. There is no acceleration in the horizontal direction which means that the velocity of the particle in the horizontal direction remains constant.

Let us consider a ball projected at an angle θ with respect to horizontal x-axis with the initial velocity u as shown below:

Projectile Motion

The point O is called the point of projection; θ is the angle of projection and OB = Horizontal Range or Simply Range. The total time taken by the particle from reaching O to B is called the time of flight.

For finding different parameters related to projectile motion, we can make use of different equations of motions:

Projectile Motion

Total Time of Flight: Resultant displacement (s) = 0 in Vertical direction. Therefore, by using the Equation of motion:

gt2 = 2(uyt – sy) [Here, uy = u sin θ and sy = 0]

i.e. gt2 = 2t × u sin θ

Therefore, the total time of flight (t):

Projectile Motion

Horizontal Range: Horizontal Range (OA) = Horizontal component of velocity (ux) × Total Flight Time (t)

R = u cos θ × 2u×sinθg

Therefore in a projectile motion the Horizontal Range is given by (R):

Projectile Motion

Maximum Height: It is the highest point of the trajectory (point A). When the ball is at point A, the vertical component of the velocity will be zero. i.e. 0 = (u sin θ)2 – 2g Hmax [s = Hmax , v = 0 and u = u sin θ]

Therefore in a projectile motion the Maximum Height is given by (Hmax):

Projectile Motion

The equation of Trajectory: Let, the position of the ball at any instant (t) be M (x, y). Now, from Equations of Motion:

x = t × u cos θ . . . . . . (1)

y = u sin θ × t – 12×t2g. . . . . . (2)

On substituting Equation (1) in Equation (2):

Projectile Motion

This is the Equation of Trajectory in a projectile motion, and it proves that the projectile motion is always parabolic in nature.

More About Projectile Motion:

Projectile motion is a type of two-dimensional motion or motion in a plane. It is assumed that the only force acting on a projectile (the object experiencing projectile motion) is the force due to gravity.

But how can we define projectile motion in the real world? How are the concepts of projectile motion applicable to daily life? Let us see some real-life examples of projectile motion in two dimensions.

All of us know about basketball. To score a basket, the player jumps a little and throws the ball in the basket. The motion of the ball is in form of a projectile. Hence it is referred as projectile motion. What advantage does jumping gives to their chances of scoring a basket? Now apart from basketballs, if we throw a cricket ball, a stone in a river, a javelin throw, an angry bird, a football or a bullet, all these motions have one thing in common. They all show projectile motion. And that is, the moment they are released, there is only one force acting on them- the gravity. It pulls them downwards, thus giving all of them an equal impartial acceleration of.

It implies that if something is being thrown in the air, it can easily be predicted how long the projectile will be in the air and at what distance from the initial point it will hit the ground. If the air resistance is neglected, there would be no acceleration in the horizontal direction. This implies that as long as a body is thrown near the surface, the motion of the body can be considered as a two-dimensional motion, with acceleration only in one direction. But how can it be concluded that a body thrown in air follows a two-dimensional path? To understand this, let us assume a ball that is rolling as shown below:

Projectile Motion

Figure 1 Motion in one dimension
Now, if the ball is rolled along the path shown, what can we say about the dimension of motion? The most common answer would be that it has an x-component and a y-component, it is moving on a plane, so it must be an example of a motion in two dimensions. But it is not correct, as it can be noticed that there exists a line which can completely define the motion of the basketball. Thus, it is an example of motion in one dimension. Therefore, choice of axis does not alter the nature of the motion itself.

Motion-in-Plane

Figure 2 Motion in Plane
Now, if the ball is thrown at some angle as shown, the velocity of the ball has an x-component and component and also a z-component. So, does it mean that it is a three-dimensional motion? It can be seen here that a line cannot define such a motion, but a plane can. Therefore, for a body thrown at any angle, there exists a plane that entirely contains the motion of that body. Thus, it can be concluded that as long as a body is near the surface of the Earth and the air resistance can be neglected, then irrespective of the angle of projection, it will be a two-dimensional motion, no matter how the axes are chosen. If the axes here are rotated in such a way that, then and can completely define the motion of the ball as shown below:

Motion in Two Dimensions

Figure 3 Motion in two dimensions
Thus, it can be concluded that the minimum number of coordinates required to completely define the motion of a body determines the dimension of its motion.

Stay tuned with Byju’s to learn more about projectile motion and its applications.


Practise This Question

A particle has initial velocity as ux i + uyj and acceleration as -ay. Find the maximum Y that it reaches?