Sir, 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.