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Motion and Rest: Definition ,Concept, How are they relative

Motion and rest are relative term and they are often used in our daily life , science etc. Clear Understanding of motion and rest is very important for solving kinematics questions. Even motion and rest principle might be used also while solving problems in Electricity and magnetism also

What is motion

An object is said to be in state of motion if its position changes with the passage of time.

What is Rest

If the position of the object does not change with the passage of time then the object is said to be at rest.

We have now define what is motion and to describe it we consider simplest form of motion that is motion of any object in one dimension for example car moving on the road. To determine the position of car at different times , we could measure its distance from starting point and record all these observations. These observations can be recorded in form of a table or we can draw a distance versus time graph for the motion of the car and see how it is moving.

Motion and Rest are Relative

In our daily life apart from given example of moving car we see a lot of objects moving around and we might be interested in describing them. I would like to make a point that both rest and motion are relative terms by this i mean that there is nothing that we would call as absolute rest or absolute motion.

For getting insight consider an example of a moving train. Two people sitting inside the moving train are at rest with respect to each other but they are moving for a person watching them standing outside the train say on the road.

Thus an object which is at rest with respect to on observer could appear to be moving with respect to another observer. So it is just a matter of choice of frame of reference which we use to observe a particular object.

Frame of Reference

To describe the motion of an object we (or observer) must specify a frame of reference relative to which motion of the object under consideration can be analyzed.

A frame of reference is a co-ordinate system and a clock fixed to a reference body as for location of an event in a frame we generally require the position and time of occurrence of the event and for this purpose we need four co-ordinates (x,y,z,t) . The reference frame used for this purpose is called space-time frame of reference. Simplest frame of reference to use is Cartesian co-ordinate system which is rectilinear. Co-ordinate systems can also be curvilinear. Which co-ordinate system we use for our problem depends on the symmetry and the formulation of the problem so that we can find the simple solution of our problem,

Frame of reference are of two types:

  1. Inertial frame of reference
  2. Non-Inertial frame of reference

1. Inertial frame of reference:-

If no external force is acting on the particle then the frame of reference with respect to which the particle seems to be moving at constant velocity or with no acceleration is known as inertial frame of reference. Since no external force is acting on the particle we have

Therefore, acceleration

or, velocity v=constant.

Clearly in absence of external force the particle will move with a constant velocity in the frame of reference. Therefore for inertial frame of reference Newton’s first and second laws of motion are always valid. By analysis it is found that such frames are either at rest or move with constant velocity. Hence for an observer at rest or moving with constant velocity frame of reference are always inertial frame of reference.

2. Non-inertial frame of reference:-

In non-inertial frame of reference Newton’s first and second laws of motion does not holds true. In non-inertial frame of reference observer or frame of reference accelerates and particle or object observed relative to them seems to be accelerated in the absence of any externally applied force.

Hope you like the explanation of the concept of motion and rest and how they are relative. Please do provide the feedback

Related Readings

How to solve Kinematics problems

Distance and Displacement

Motion in a plane

How to effectively solve One dimensional Motion Problem

How to solve motion Problems

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