When we learn projectile motion, everything seems clean and predictable: the object rises, falls, and follows a perfect curve. But have you ever wondered why the equations always work so neatly?
It’s because they’re built on a set of hidden assumptions.
In real life, a football, bullet, or basketball doesn’t behave exactly like what your textbook says. That’s because physics in the classroom simplifies the world so we can understand the core ideas before dealing with the messy details. These simplifications—called assumptions—make the math easier and the motion idealized.
Let’s uncover these 7 key assumptions and understand why we use them, and what would happen if we didn’t.
1. No Air Resistance
We assume the projectile faces no drag from the air. This makes the motion smooth and symmetric—a perfect parabola.
But in real life, air resistance depends on the object’s shape, size, speed, and direction. This causes the projectile to slow down, especially in the horizontal direction.
The result? A distorted path:
- The horizontal velocity decreases over time.
- The descent becomes steeper than the ascent.
- The overall path is asymmetric, not a perfect parabola.
To model this realistically, we’d need to solve non-linear differential equations—topics from fluid dynamics and advanced calculus. These are best handled through numerical methods or computer simulations, which are far beyond the high-school level.
2. Constant Gravity (g = 9.8 m/s²)
We assume gravity stays the same throughout the motion. In reality, gravity slightly weakens with height.
Why do we treat it as constant?
The formula for real gravity is:
But for everyday projectiles (a few meters to hundreds), the change is so small that we can ignore it. Keeping g = 9.8 m/s² makes calculations simple and accurate enough.
3. Flat Earth Approximation
In projectile motion problems, we assume the Earth is flat. Why? Because most projectiles—like a ball, a rock, or even a cannon shell—don’t travel far enough for Earth’s curvature to matter.
For small distances (a few hundred meters to maybe 10 kilometres), the ground appears flat and level. This lets us use straight-line horizontal motion without curving it along the surface of the Earth. If we didn’t make this assumption, we’d have to use spherical coordinates and account for the Earth’s curve—and that’s far more complex than needed at this level.
So, unless we’re talking about missiles, satellites, or long-range artillery, this flat-Earth assumption is totally reasonable for school-level physics.
4. Two-Dimensional Motion
In school physics, we treat projectile motion as happening in just two dimensions—horizontal and vertical (the x-y plane). That means we only track how far the object moves across and how high it goes.
This simplifies things a lot. We can split the motion into two parts:
- Horizontal motion → constant velocity
- Vertical motion → acceleration due to gravity
But in real life, objects may also move sideways (z-direction), especially if they spin or if wind acts from the side. That would make it a three-dimensional problem, which needs vectors, angles in space, and much more advanced math.
So, unless stated otherwise, we stick to 2D motion. It keeps the problem clean, solvable, and clear.
5. Horizontal and Vertical Motions Are Independent
One of the most powerful ideas in projectile motion is this: the horizontal and vertical motions happen independently of each other. The only connection between them is time.
Horizontally, there’s no force acting (since we’re ignoring air resistance), so the object moves with a constant velocity.
Vertically, gravity acts downward—so the object slows down as it rises, and speeds up as it falls.
This lets us treat both motions separately, using simple kinematic equations. We don’t have to combine them into one complicated path. Time acts as the link that helps us figure out what’s happening in each direction at any moment.
In real-world physics, motions aren’t always this clean—wind, friction, and spinning objects often mix the two. But for our level, this assumption makes it possible to understand and solve problems clearly.
6. No Spin or Rotation
In our projectile motion model, we assume the object does not spin or rotate as it moves through the air. Why? Because spin creates extra forces that change the path in ways basic physics doesn’t cover.
For example, spinning balls can curve mid-air due to the Magnus effect—a force caused by the interaction of spin and air. This effect is why a football or baseball can “bend” or “swerve” during flight.
If we included spin, we would have to use complex fluid dynamics and aerodynamic models, which go beyond the scope of high school physics.
So, by ignoring spin and rotation, we keep the problem simple and focused on the main forces at work—gravity and initial velocity.
7. No Rebound or Bounce
In projectile motion problems, we assume the object stops moving once it hits the ground or any surface. That means no bouncing or rebounding unless the question specifically says otherwise.
Why do we assume this? Because bouncing involves collision physics, which includes concepts like energy loss, elasticity, and contact forces—topics that are more advanced and separate from basic projectile motion.
If we had to consider bounces, we’d need to analyze each collision using different rules, making the problem much more complicated.
So, for now, the path of the projectile is only valid up to the first contact with a surface.
Closing Thoughts
These assumptions are not lies—they’re approximations. They help us focus on the core ideas of projectile motion without getting lost in complicated math. Without these simplifications, we wouldn’t be able to solve problems using neat formulas and parabolic paths.
But now that you know what’s hidden beneath the surface, you’re better prepared. In real-world situations—whether it’s sports, engineering, or spaceflight—these assumptions often break down. And that’s when deeper physics begins.
Next time, we’ll dive into how to solve projectile motion questions using the tools these assumptions give us.
