• We know that nothing is either in absolute motion or at absolute rest. Absolute means something that is universally valid or totally true to the fact. I had a misconception in the childhood that the sun is at rest. Only we, the planets, are revolving around it. But that’s not true. The sun also revolves around the milky way galaxy. It’s not at absolute rest.
  • So, what’s the point of mine to talk about all this? My point is as physics students we should have clarification that we can’t use the words “Rest” and “Motion” all alone. We have to use like that car is moving. No. That’s partially correct. With respect to who? With respect to who the car is moving? That’s the question. That’s important. You can’t skip that.
  • The man sitting inside a bus says he’s at rest. But another man standing on the other end of the street says that the man inside the bus is moving. The man inside the bus is at rest w.r.t. the bus, and for the man on the streets, the man inside the bus is moving. So, the main factor in all this is a reference frame. It’s a very crucial term in physics. We judge whether an object is moving or at rest taking a reference frame into consideration.


  • There are two types of frames of reference.
  • Inertial frame
  • Non-inertial frame

I. Inertial Frames:

 A frame is usually a coordinate system with respect to which we measure the movement or motion of the body. An inertial frame is a coordinate system in which Newton’s first law of motion holds good. In this coordinate system, a body continues its state of rest or uniform translation(in a straight line) motion as long as no external force is applied on it. All reference frames moving in uniform motion with other inertial motion are also inertial frames. When I say uniform motion, it means the motion is non-accelerated. For instance, when we are travelling inside a car on an empty highway, we are constantly travelling with a high speed, making the car an inertial frame of reference.

II. Non-Inertial Frame:

  A non-inertial frame is a coordinate system in which Newton’s laws of motion don’t hold good. A non-inertial frame is either a frame having uniform linear acceleration or a frame rotating uniformly. As a uniformly rotating frame has a centripetal acceleration, it’s also an example of a non-inertial frame.

Earth-A Non-Inertial Frame:

Though we can’t feel the movement of the earth in our daily life, it has its motion around the sun as well as its spin around its own axis. So, the earth is usually taken to be a satisfactory inertial frame. But in reality, it’s a non-inertial frame.

Distant Stars as Inertial Frames:

We consider the very distant stars as inertial frames. You will ask how can they be; as they must also move around some galaxy or something. But their velocity from this far off distance on our planet appears to be really negligible. So, any rigid body that is far off from our planet in deep interstellar space can be taken as an inertial frame of reference.

Vitality of Frames of Reference:

So, the use of frames of reference is very vital in understanding the relative motion of bodies. The concept of an inertial frame is crucial as it’s in respect of such a reference frame that all the fundamental laws of physics have been formulated. It has really great use in the special theory of relativity. Non-inertial frames have an instrumental role in the General Theory of Relativity too.

Author: Som Abhisek.

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