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Please say whether these 2 problems are right.

1) A body of mass 2kg moving on a horizontal surface with an initial velocity of 4 m/sec comes to rest after 2 seconds. If one wants to keep this moving on the same surface with same velocity, what is the force that is needed to be applied?

I solved it in the following way:

Acceleration = (0 – 4)/2

= -2m/sec^2

Retarding force = mass x retardation

= 2 x 2

= 4 Newton

So if a force equal to the retarding force is applied, the body will move with a constant velocity of 4 m/sec^2. But the book answer is 2 Newton. What is your opinion Sir?

2) A splash is heard 4.28 seconds after a stone is dropped into a well 78.4 meters deep. What is the velocity of sound?

I solved it in the following way:

Time taken by the stone to reach the well = [2H/g]^1/2

(Here H is depth of well & g = 9.8 m/sec^2)

= 4 seconds

Time taken by sound to reach the observer = 4.28 – 4

= 0.28 seconds

Velocity of sound = 78.4/0.28

= 280 m/sec

But the book answer is 341 m/sec. What is your opinion?I think the book answer is wrong.

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# Homework Help: One dimensional problem

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