ï»¿ A man walks on a straight road from his home to a market 2.5 km away with aÂ speed of 5 km h-1. Finding the market closed, he instantly turns and walks backÂ home with a speed of 7.5 km h-1. What is the(a) magnitude of average velocity, and(b) average speed of the man over the interval of time(i) 0 to 30 min, (ii) 0 toÂ 50 min, (iii) 0 to 40 min ? [Note: You will appreciate from this exercise why itÂ is better to define average speed as total path length divided by time, and notÂ as magnitude of average velocity. You would not like to tell the tired man onÂ his return home that his average speed was zero !] from Physics Motion in Straight Line Class 11 Manipur Board

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Motion in Straight Line

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A man walks on a straight road from his home to a market 2.5 km away with aÂ speed of 5 km h-1. Finding the market closed, he instantly turns and walks backÂ home with a speed of 7.5 km h-1. What is the
(a) magnitude of average velocity, and

(b) average speed of the man over the interval of time
(i) 0 to 30 min, (ii) 0 toÂ 50 min, (iii) 0 to 40 min ?
[Note: You will appreciate from this exercise why itÂ is better to define average speed as total path length divided by time, and notÂ as magnitude of average velocity. You would not like to tell the tired man onÂ his return home that his average speed was zero !]

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What is motion?

When an object changes it's position with time, with respect to it's surroundings, the object is said to be in motion.
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What is the most essential thing to study the motion of a body?

The most essential thing to study the motion is reference frame. Rest and motion are relative terms. It means that an object in one situation can be in rest but in another situation, the same object will be in motion.
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What is rest?

When an object does not change its position with time, with respect to it's surroundings. In such a case the object is said to be at rest.
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Is absolute rest or absolute motion possible?

Absolute rest and absolute motion is not possible because there is no point in the universe which is at rest and can be taken as point of reference. The planet Earth is in continuous motion.
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Can a body be always at rest in some frame?

Yes, a body is always at rest in a frame which is fixed with the body itself.
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