Study Guides/Physics/Equation of Trajectory
Study Guide · Physics

What is the Equation of Trajectory in Projectile Motion?

When you kick a football or fire a cannonball into the air, the object flies forward and upward, reaches a peak, and then curves back down to the ground. This physical curved path is known as a Trajectory.

In classical physics (kinematics), the mathematical equation that defines this exact curved path is called the Equation of Trajectory. By looking at the math, we can prove that the path of any projectile is a perfect Parabola.

Question (Click to Flip)

What is the equation of trajectory?

Answer

It is the mathematical formula that describes the exact flight path of a projectile: y = x·tan(θ) - [gx²] / [2u²·cos²(θ)].

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Key Facts

Definition: The mathematical path drawn by a flying projectile in space.

Shape of Path: A Parabola.

Core Concept: It links the horizontal distance (x) and vertical height (y) without needing the Time variable (t).

Key Constants: It relies heavily on the initial launch angle (θ) and initial launch speed (u).

The Mathematical Equation

The standard Equation of Trajectory relates the vertical height (y) of the projectile to its horizontal distance (x) at any given moment in time, completely removing the variable of 'time' (t) from the equation.

The final derived formula is: y = x · tan(θ) - [g · x²] / [2 · u² · cos²(θ)]

Where:

  • y = Vertical position (height)
  • x = Horizontal position (distance traveled forward)
  • θ = The initial angle of projection (the angle you kicked the ball at)
  • u = The initial velocity (speed of the kick)
  • g = Acceleration due to gravity (9.8 m/s²)

Why is it a Parabola?

In mathematics, the standard equation for a parabola is y = ax - bx². If you look closely at our physics equation, it perfectly matches this exact mathematical structure.

  • The term tan(θ) acts as the constant 'a'.
  • The massive block g / [2u² cos²(θ)] acts as the constant 'b'. Because the 'x' variable is squared (x²) while the 'y' variable is not, this definitively proves that the flight path of any projectile under gravity is a parabolic curve.

Why is this Equation Useful?

This specific equation is incredibly powerful for solving complex physics problems because it does not require you to know the 'time of flight'. If a military engineer knows the angle of a cannon (θ) and the speed of the shell (u), they can use this equation to find exactly how high the shell (y) will be when it is exactly 500 meters away (x).

Questions and Answers

What is the equation of trajectory?+

It is the mathematical formula that describes the exact flight path of a projectile: y = x·tan(θ) - [gx²] / [2u²·cos²(θ)].

What is the shape of the trajectory of a projectile?+

The trajectory of a projectile moving freely under gravity is always a perfect parabolic curve.

Why does the equation of trajectory not have 't' (time) in it?+

During derivation, the equation for time (t = x / u·cosθ) is deliberately substituted into the 'y' equation to completely eliminate the time variable, allowing us to directly compare height (y) against distance (x).

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