A frame of reference is essentially a set of coordinate axes used to define an object’s position in two- or three-dimensional space. It can also be defined as a set of coordinates used to record the position and time of events. There are two types of frames of reference: inertial frame of reference and non-inertial frame of references.
Since we’ve already covered the first type—inertial frames of reference—in a previous post, we’ll now move on to the second type: non-inertial frames of reference.
A non-inertial frame of reference is a frame that is accelerating relative to an inertial frame. Unlike an inertial frame, Newton’s laws don’t strictly apply here without adding extra “fictitious” forces to explain what’s happening. In simpler terms, when an object seems to accelerate even though no visible external force is acting on it, the frame of reference you’re observing from is non-inertial.
In everyday life, there are many situations that act as non-inertial frames of reference. Here are 10 of the most common and relatable examples:
1. Braking Car
When you’re driving and suddenly spot a pothole, you slam on the brakes, and it feels as if your body is being thrown forward. In reality, nothing is actually pushing you. What’s happening is that the car is decelerating, making the interior of the car a non-inertial frame. Your body wants to keep moving at the original speed (thanks to inertia), so relative to the slowing car, you appear to lurch forward.
2. Airplane during Takeoff
When a plane accelerates down the runway, passengers feel themselves pressed back into their seats. Inside the cabin—the non-inertial frame—this feels like a backward force, even though it’s just the plane moving forward faster and faster. Pilots and engineers account for this acceleration when designing seatbelts and safety features to keep passengers secure.
3. Roller Coaster Loop
When you’re inside a roller coaster car racing through a loop, you feel as if you’re being pushed into your seat even though the car is actually turning and accelerating. From your perspective inside the moving car—the non-inertial frame—it feels like a mysterious “force” is pressing you outward, but in reality, it’s the car’s rapid change in direction that creates this sensation. The accelerating frame makes you experience what are called fictitious forces, like the outward push of “centrifugal force.”
4. Spinning Merry-Go-Round
If you sit on a spinning merry-go-round, you’ll feel like you’re being pulled outward, away from the center. That outward tug is a fictitious force caused by the platform’s rotation. Because the merry-go-round is constantly changing direction, anyone on it is in a non-inertial, rotating frame, where straight-line motion appears to bend outward.
5. Elevator Starting Up or Down
When an elevator suddenly starts moving upward, you might feel heavier for a moment; when it starts downward, you feel slightly lighter. These sensations happen because the elevator is accelerating, making its interior a non-inertial frame. The floor pushes on you differently depending on the direction and speed of the acceleration, creating that fleeting change in weight.
6. Bus Making a Sharp Turn
Ever been on a bus that suddenly turns a corner? You feel like you’re being pushed sideways, even though nothing is actually pushing you. The bus is changing direction quickly, which makes it a non-inertial frame. Your body wants to keep moving straight, but inside the turning bus it seems like an invisible sideways force is at play.
7. Spinning Office Chair
If you sit on a swivel chair and give yourself a strong spin, you might feel dizzy or like you’re being pulled outward. That feeling is caused by the rotation, which turns your chair into a non-inertial frame. Your body is trying to move in a straight line, but the spinning seat keeps changing your direction, creating that strange “pull.”
8. Earth Itself (Because of Rotation)
Even though we don’t usually feel it, Earth is spinning on its axis. This rotation means the surface of the Earth is technically a non-inertial frame. We don’t notice the acceleration most of the time, but scientists must account for Earth’s rotation when studying things like weather patterns or when launching rockets into space.
9. Racing Car Accelerating Out of a Curve
When a race car speeds up as it comes out of a turn, the driver feels pushed back into the seat. The car is both turning and accelerating, so it’s a perfect example of a non-inertial frame. Inside the car, the driver has to fight against forces that seem to appear from nowhere, which is why they wear special harnesses to stay secure.
10. Swinging Pendulum Ride at a Carnival
Think of those giant pendulum rides at amusement parks that swing back and forth while sometimes spinning. Riders feel forces that keep changing direction—forward, backward, and sideways. The ride is constantly accelerating, making the cabin a classic non-inertial frame where you feel like you’re being tossed around by invisible hands.