Homework Help: Newton Problem (2nd Law of Motion)

1. Sep 8, 2006

greeenmonke

the problem: a plane with a mass of (3.5X10^5)kg is landing. If the speed is 27 m/s and the net braking force is (4.30X10^5) N then a) what is the speed 7.5 sec later? and b) how far has it traveled in this time?

this is how i started, because speed is distance/time i felt that it would be better for me to fine the the distance first.
so: with the given information i solved for the acceleration using F=MA.
the a turned out to be 5.282 m/s^2. from here i used the eq: x=x0+ v0t + .5a(t^2). and this gave me the distance of 53.94 m.
then i used this to solve for a which was just 53.94/7.5= 7.192m/s.

i am just wonder if my thought process is correct. in the problem it is not stated the the 27 m/s is the initial velocity so i am wondering if it is ok if i used it as such. if anyone could shine some light on the problem or tell if i did it correctly i would appreciated.

2. Sep 8, 2006

steve23063

What you did is perfect. 27m/s IS the intial velocity. Initial velocity just means the velocity at some time t=0, but you get to pick when that time is. In this problem you essentially chose t=0 to be the instant when the plane is moving 27m/s.

3. Sep 8, 2006

greeenmonke

ahh thank you so much.

4. Sep 10, 2006

sweetshelly

how did you get the accerleration=5.282 m/s^2?
I try a= F/m but got a different answer: 1.22m/s^2