Question

a) When a body is said to be at rest ?
b) when a body said to be in motion ?


Please explain it clearly and nicely

Answer 1

When a body's position doesn't change relative to a particular frame of reference, with respect to other objects in its surrounding, it's said to be at rest. It's all the three coordinates stay the same.
Example, your grandma sitting for hours on the rocking chair, JK that would be a bad example. Well, an appropriate example is ME getting punished at school to stand outside the class holding MY ears

A body is said to be in motion if it's position changes with time, with respect to other objects in that frame of reference. One or more coordinates of the body changes in motion.
Example, suppose MY teacher gives ME the punishment of taking 5 rounds of the playground, you run, I move - I AM in motion...

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