Problem on kinematics of body falling from space

[chiraganand]

Problem goes like this.. body falls toward a planet with acceleration of
-g0*R^2/s^2. where R is the radius of planet 4000 miles, g0 is 30 ft/s^2, s is the distance of the body from the planets center. the initial distance of the body from the surface is 400 miles. find the impact velocity.

Pretty straight forward problem. i solved by substituting all the value in equation of acceleration and then finding impact velocity by equation v^2=u^2-2as. But the problem is i am getting the impat velocity as 6933 mph . The textbook answer is different. Could someone please verify and let me know if i am right or wrong
Would greatly appreciate the help.
Thanks

 

[Simon Bridge]

i solved by substituting all the value in equation of acceleration and then finding impact velocity by equation v^2=u^2-2as.

... that equation only holds for constant acceleration - but the acceleration is not constant.

 

[chiraganand]

... that equation only holds for constant acceleration - but the acceleration is not constant.

yes just realized that.. thanks for the help.. used the varying acceleration equation and got the answer

 

[Simon Bridge]

Sometimes your mind doesn't connect the dots until you tell someone else about it :)
Well done.

 

[chiraganand]

Ya happens more than often to me.. :) thanks anyways...