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Motion in Straight Line

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Physics Part I

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Class 10 Class 12
Suggest a suitable physical situation for each of the following graphs.

(a) The given position-time graph shows that initially a body was at rest. Gradually, its velocity increases with time and attains an instantaneous constant value. Then, the velocity reduces to zero with an increase in time. Then, its velocity increases with time in the opposite direction and acquires a constant value.

A physical situation analogous to this graph arises when a football (initially kept at rest) is kicked and gets rebound from a rigid wall so that its speed gets reduced. Then, it passes from the player who has kicked it and ultimately gets stopped after sometime. 

(b) In the given velocity-time graph, the sign of velocity changes and its magnitude decreases with a passage of time. A similar situation arises when a ball is dropped on the hard floor from a height. It strikes the floor with some velocity and upon rebound, its velocity decreases by a factor. This continues till the velocity of the ball eventually becomes zero.

(c) The given acceleration-time graph reveals that initially the body is moving with a certain uniform velocity. Its acceleration increases for a short interval of time, which again drops to zero. This indicates that the body again starts moving with the same constant velocity. A similar physical situation arises when a hammer moving with a uniform velocity strikes a nail. 

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. 

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. 

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. 

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. 

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.