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Frames of Reference are abstract coordinate systems whose origin, orientation, and scale are defined by a set of reference points (geometric points). The position of these reference points is identified both mathematically and physically. A frame of reference is required to measure various quantities such as displacement, velocity, and acceleration. The frame of reference can be of two types: Inertial and Non-inertial.
Also Read: Motion in a Straight Line
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Key Terms: Frame of Reference, Inertial Frame, Non-Inertial Frame, Velocity, Displacement, Acceleration
What is Frame of Reference?
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In physics and astronomy, frames of reference refer to an abstract coordinate system whose orientation, origin, and scale are specified by a set of reference geometric points.
Frames of reference have physical reference points that uniquely fix the coordinate system and standardize measurement within that frame.
Frame of Reference
Example of Frames of ReferenceSuppose a train is moving and a person is sitting inside the train watching a tree pass by. In this case, the train is the frame of reference and the tree appears to be moving. If we consider the opposite of it, a person on the platform will look at the tree as a stationary object. It will be his/her frame of reference. Both these observations are correct yet different due to the different frames of reference. |
Types of Frame of Reference
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According to Newton's law, if an object’s net force is zero then its acceleration is also zero. In this case, the body might be in rest or uniform motion with a constant velocity vector. Different frames of reference were observed.
The frame of reference where the first law of motion is valid and can be directly applicable becomes the Inertial Frame of Reference. And those frames of reference where Newton's First Law of Reference is not directly applicable, it is Non-Inertial Frame of Reference.
There are two types of frame of reference that are as follows:
- Inertial Frame of Reference
- Non–Inertial Frame of Reference
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Inertial Frame of Reference
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Inertial Frame of Reference is a frame of reference attached to a stationary object or an object moving with constant velocity.
In this frame, Newton's First Law of Motion is directly applicable. In the Inertial Frame of Reference, acceleration is zero, in this case, the object can be at rest or in uniform motion and the object is at constant velocity.
Inertial Frame of Reference
Newtonian Mechanics will be applicable over here. In an inertial frame of reference, if no force is acting on a body, it either remains in uniform motion or stays at rest.
The inertial frame moves with a constant velocity or stays at rest with reference to the initially assumed frame of reference. For example, a rocket moves at a constant velocity relative to the Earth.
Non-Inertial Frame of Reference
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A frame of reference where Newton's first law of motion is not directly applicable is called the Non-Inertial Frame of Reference. So as the First law of motion is not applicable here, we need some force by which Newton's first law of motion can be valid in non Inertial Frame of Reference.
In this case, a pseudo force is applied and it helps to validate Newton's first law of motion in a Non-Inertial Frame of Reference. In these Frames of Reference, the acceleration is not zero; therefore, the Non-Inertial Frame of Reference has a certain acceleration.
There are several ways to imagine the pseudo-motion to make Newton’s first law of motion valid. They are as follows:
- The frame could travel in a straight line, but by slowing down or speeding up.
- The frame could travel on a curved surface at a steady speed.
- The frame could travel on a curved path and can be speeded up or slowed down.
Frame of Reference
Example of Frames of ReferenceWhen we see a ball rolling on the street, then we can say that the ball is moving as the frame of reference is there on the streets, whatever may be there on the side of the roads or the earth. All measurements of motion will be compared to the frame of reference. Therefore the best example of a frame of reference is the earth, even if it keeps on moving. A sitting person on Earth is at rest with respect to the Earth’s frame of reference while it is in motion with respect to the moon. Therefore, if we take Earth as our frame of reference then the moon is in accelerated motion with respect to the Earth and it becomes our non-inertial frame of reference. |
Things to Remember
- A frame of reference is required to measure various quantities such as displacement, velocity, and acceleration.
- A frame of reference refers to an abstract coordinate system whose orientation, origin, and scale are specified by a set of reference geometric points whose positions are identified both mathematically and physically.
- A frame of reference attached to a stationary object or an object moving with constant velocity is called the Inertial Frame of Reference.
- In the Inertial Frame of Reference, Newton’s first law of motion is applicable directly.
- A frame of reference where Newton's first law of motion is not directly applicable is called the Non-Inertial Frame of Reference.
- In that case, we use pseudo forces to make Newton’s first law valid.
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Sample Questions
Ques. How many types of frames of reference are there? (1 mark)
Ans. There are two types of frames of reference. They are the Inertial Frame of Reference and the Non-Inertial Frame of Reference.
Ques. What is the Inertial Frame of Reference? (2 marks)
Ans. A frame of reference attached to a stationary object or an object moving with constant velocity is called the Inertial Frame of Reference and in this Newton's First law of motion is directly applicable. In the Inertial Frame of Reference, the acceleration is zero in this case the object can be at rest or maybe at uniform motion and the object is at constant velocity.
Ques. What is the Non-Inertial Frame of Reference? (3 marks)
Ans. A frame of reference attached to an accelerated object is called the Non-Inertial Frame of Reference. So we get to know that the First law of motion is not applicable in non Inertial Frame of Reference, so we need some force by which Newton's first law of motion can be valid in non Inertial Frame of Reference, in this case, pseudo force is applied and it helps to make Newton's first law of motion valid in Non-Inertial Frame of Reference. In these Frames of Reference, the acceleration is not zero. Here Newton First law is not applicable directly we need to take the help of pseudo forces to make it applicable.
Ques. What are Pseudo Forces? (2 marks)
Ans. We know that the first law of motion is not applicable in the Non-Inertial Frame of Reference, so we need some force by which Newton's first law of motion can be valid there. In this case, pseudo force is applied and it helps to make Newton's first law of motion valid in non Inertial Frame of Reference.
Ques. Give one example of a Frame of Reference? (2 marks)
Ans. A train is moving and a person is sitting inside the train watching a tree pass by, in that case, the train is the frame of reference and the tree appears to be moving while if we consider the opposite of it, a person on the platform observes a stationary tree as for them, the stationary platform is their frame of reference. Both these observations are correct yet different due to the different frames of reference.
Ques. Following two equations represents the x – t relation for the motion of objects.
(i) \(x (t) = x(0) + v(0)t + \ (\frac{1}{2})\) at2
(ii) \(x(t) = v(0)t + \ (\frac{1}{2})\) at2
What is the difference between them? (1 Mark)
Ans. The first equation is a more general form of motion as it contains information regarding the initial position of the object.
Ques. A car travelling at a speed of 60 km h-1 on a straight road is ahead of a scooter travelling at a speed of 40 km h-1. How would the relative velocity be altered if the scooter is ahead of the car? (4 Marks)
Ans. vc = speed of car = 60 kmh-1
vs = speed of scooter = 40 kmh-1
vcs = relative velocity of car w.r.t. scooter
= vc – vs
= 60 – 40
= 20 kmh-1
Similarly vsc = relative velocity of scooter w.r.t. car
= vs – vc
= 40 – 60
= – 20 kmh--1
Therefore, the magnitude of the relative velocity is the same in both cases but the direction of relative velocity is reversed if the scooter is ahead of the car.
Ques. A ball thrown upward reaches a height and comes bad downward. Out of the following statements, which one is true for displacement, velocity, and acceleration.
(a) It varies continuously but never changes the sign.
(b) It varies continuously with the maximum, in the beginning, being zero at the top.
(c) It remains constant throughout the course of the journey.
(d) It only changes the sign when the ball is at the top. (3 Marks)
Ans.
- Statement (a) is true for displacement as it varies from starting
- Statement (b) is true for velocity as it is maximum at the time o projecting the ball and becomes zero at the highest point i.e. top.
- Statement (c) is true for acceleration as it is always constant throughout the course of the journey and is equal to ‘g’.
Ques. To deal with physical phenomena, we consider objects even as big as Sun a point objects. Can you name physical phenomena in which Earth cannot be taken as a point object? (2 Marks)
Ans. The occurrence of solar or lunar eclipse does not allow Earth to be taken as a point object otherwise the phenomena cannot be explained.
Previous Year Questions
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