Dynamics - supertanker coasting to a stop

[azukibean]

Dynamics -- supertanker coasting to a stop

1. Problem
Assuming constant negative acceleration, how far does a supertanker of a mass of 250000 metric tons travel while coasting to a stop from an initial speed of 30 km/h in 21 min?
What force is supplied by fluid friction of the water while the tanker is stopping?

2. Relevant formulas
S-S0 = (V+V0)t/2
ma = sum (F)
(1/2)at^2+v0t+S0=0
v=at+v0

3. Work I Did
250k met ton = 250,000,000 kg
30 km/h = 500 m/s
21 min = 1260 s
0 = 1260a + 500
a = -500/1260
(0+500)/2*1260 = 315 km (part a)
ma = -F(friction)
1000*(-50/126); Ff = 396.82 N (part b)

a: 5.25 km
b: 1.65*10^6 N

 

[CWatters]

30km/h is not 500 m/s !

 

[azukibean]

Thanks!
Using the same equations as I did in part a I got 5250 m -> 5.25 km; a = -5/756; v0 = 50/6.
Part B I used 250,000,000 for the mass instead of 1000 kg and got the right answer.
This is kind of embarrassing. I must've stared at the problem, confused, for quite a while.