In this explainer, we will learn how to find the horizontal and vertical components of the velocity of a projectile and analyze and solve problems associated with projectile motion at any angle.
Suppose a particle is projected from a flat horizontal plane, at some angle above the horizontal, with an initial velocity of mβ sβ1.
To analyze this motion, it can be very useful to split the particleβs velocity up into its horizontal and vertical components.
The horizontal and vertical components of the velocity are given by
We write for the unit length vector in the horizontal direction and for the perpendicular unit length vector in the vertical direction. Then, we can organize this information into a single velocity vector expressed in terms of the unit vectors and :
On the other hand, if we are given a projectileβs initial velocity vector, we can recover its initial speed and angle of projection.
If a projectile has an initial velocity vector , then applying the Pythagorean theorem gives us the initial speed as
Looking at the diagram of horizontal and vertical components of the initial velocity, we can see that , and so the angle of projection is
Let us practice writing down the initial velocity in vector form.
Example 1: Finding the Vector Form of the Initial Velocity of a Vector
A particle is projected from a point on a horizontal plane with an initial velocity of 39 mβ sβ1 at an angle of above the horizontal, where . Express the initial velocity as a vector in terms of and , the horizontal and vertical unit vectors in the vertical plane.
Answer
The horizontal and vertical components of the initial velocity vector are given by where is the angle of projection. In this question, we are not given the value of , but rather its tangent. However, this is enough information to write down and , which are what we need here. Given that , we can sketch the following triangle.
The Pythagorean theorem gives us the length of the hypotenuse as
Therefore, and . Furthermore, we are told that the initial velocity of the particle is 39 mβ sβ1, so our initial velocity vector is
Having decomposed an initial velocity into horizontal and vertical components, let us now see an example of the opposite procedure: recovering the initial velocity and angle of projection from a velocity vector.
Example 2: Finding the Initial Speed and Angle of Projection of a Projectile given Its Initial Velocity Vector
- A particle is projected with velocity , where and are horizontal and vertical unit vectors. Find the initial speed of the particle approximated to one decimal place.
- Find the angle of projection of the particle approximated to one decimal place.
Answer
Part 1
If a projectile has an initial velocity vector , then the initial speed is given by and the angle of projection is
It is almost always a good idea to draw a sketch. So, let us do that as follows.
From the sketch, we can see that the initial velocity is given by approximated to one decimal place.
Part 2
Furthermore, the angle of projection of the particle is approximated to one decimal place.
Now that we know how to decompose a projectileβs velocity into horizontal and vertical components, we can use this technique to solve problems about projectile motion. To do this, we will need to recall the equations of motion. In particular, to solve problems involving the displacement of a projectile at a time , given that it has initial velocity and acceleration , we need the equation of motion
Example 3: Finding the Components of the Initial Velocity of a Projectile
A flare was fired from a flare gun. After 2 seconds, the flareβs horizontal and vertical distances from the flare gun were 22 m and 31 m respectively. Find the horizontal and vertical components of the flareβs initial velocity. Take .
Answer
We will analyze the horizontal and vertical components of the flareβs motion separately. Let us begin with the horizontal component.
We want to find the flareβs initial horizontal velocity . After its launch, there are no forces acting on the flare in the horizontal direction. This means that its horizontal acceleration is zero. We are told that the particleβs horizontal displacement is 22 m after time . We can substitute these values into the equation of motion to get
Therefore, the horizontal component of the flareβs initial velocity is .
We now analyze the vertical component of the flareβs motion. We are told that the flareβs vertical displacement is 31 m after time and that its vertical acceleration is m/s2 due to gravity. Notice that the negative sign on the flareβs vertical acceleration comes from the fact that this acceleration is in the opposite direction to the flareβs initial vertical velocity . We substitute these values into the equation of motion to get
Therefore, the vertical component of the flareβs initial velocity is .
We can use these same techniques to solve more complicated problems involving projectile motion.
Example 4: Finding the Horizontal Distance of a Particle to Reach the Ground given Its Position after a Given Time of Projection
A particle was projected from a point on horizontal ground. If, after 8 seconds, its horizontal distance from was 48 m and its height was 98 m, determine how far the particle would be from when it hits the ground. Take .
Answer
We need to tackle this question in three steps. First, we will analyze the vertical component of the particleβs motion and use the values given at time to find the vertical component of its initial velocity. Second, we will use the initial vertical velocity that we have calculated to work out the time when the particle hits the ground. Third, we will analyze the horizontal component of the particleβs motion to find its horizontal displacement when it hits the ground.
First, we need to find the vertical component of the particleβs initial velocity. We are told that the particle has a vertical displacement of 98 m after 8 s and that its acceleration due to gravity is 9.8 m/s2 in the negative vertical direction. We substitute these values into the equation of motion to get
We now want to find the time when the particle hits the ground. Let us sketch the situation.
The particle hits the ground when its vertical displacement is zero. We can see that there are two times when the particle has a vertical displacement of zero: its moment of launch and the time when it hits the ground , which we want to find. We substitute again into the law of motion using the initial velocity that we have just calculated:
We can see that this quadratic equation has two solutions: one at and the second at , when . Thus, we have
We can now analyze the horizontal component of the particleβs motion to find its horizontal displacement when it hits the ground. We first need to calculate its initial horizontal velocity . We are told that at time , the particle has a horizontal displacement of 48 m. We also know that its horizontal acceleration is 0. We substitute these values into the equation of motion to get
Finally, we substitute this horizontal velocity and the time of landing back into the equation of motion, observing that the horizontal acceleration remains as 0:
We conclude that the particle would be 63 m from when it hits the ground.
We can also solve problems that do not explicitly involve a projectileβs displacement, but rather its initial velocity , final velocity , and acceleration . For this, we will need the equation of motion
The following is an example of this type.
Example 5: Finding the Time Taken by a Projectile to Reach Its Maximum Height
Suppose a particle is projected at a speed of 21 m/s and an angle of elevation . How long will it take for the particle to reach its greatest height? Give your answer correct to two decimal places. Take .
Answer
To solve this problem, we use the observation that a projectile reaches its greatest height at the moment when it stops rising and starts fallingβthat is, when its vertical velocity is 0. In order to make use of this observation, we first need to calculate the vertical component of the particleβs initial velocity.
The vertical component of the particleβs initial velocity is given by .
We want to find the time when the particleβs vertical velocity is 0. We substitute our initial velocity into the equation of motion , observing that the acceleration due to gravity is negative as it is in the opposite direction to the particleβs initial vertical velocity:
We conclude that the particle reaches its greatest height after 1.67 s, approximated to two decimal places.
Let us finish by recapping a few important concepts from this explainer.
Key Points
- Given a projectile with initial speed m/s and angle of elevation , we can decompose its motion into horizontal and vertical components and by using the formulas
- We can organize this information into a single velocity vector expressed in terms of the horizontal and vertical unit vectors and :
- Given an initial velocity vector , we can recover a projectileβs initial speed and angle of elevation by using the formulas and
- To solve more complicated problems about projectile motion, we can use the horizontal and vertical decomposition of a projectileβs motion along with the equations of motion and
