@elena
posted by john .
thank you so much help with the problem. i have trouble solving part b of the problem you helped me with. it told me to give it as a percentage, I got 10% but it is wrong. can you please help me
Superman must stop a 190km/h train in 180m to keep it from hitting a stalled car on the tracks.
part a)If the train's mass is 3.8×10^5kg , how much force must he exert (find the magnitude)?
part b)Compare to the weight of the train (give as %).
partc) How much force(in magnitude) does the train exert on Superman?
college physics  Elena, Sunday, April 29, 2012 at 5:09pm
(a) v=190 km/h =52.8 m/s,
a =v^2/2•s =(52.8)^2/2•180 = 7.74 m/s^2,
F = m•a = 3.8•10^5•7.74 = 2.94•10^6 N.
(b) Weight = mg = 3.8•10^5•9.8= 3.76•10^6 N.
(c) F12 = F21 = >
F = 2.94•10^6 N.

nvm, i did it wrong. i already got the answer its 79% thanks

2.94•10^6 ....100%,
3.76•10^6 ..... x %
x=(3.76•10^6 /2.94•10^6)•100% =127.9% 
3.76•10^6 ....100%,
2.94•10^6 ..... x %
x=(2.94•10^6 /3.76•10^6)•100% =78%
It depends on what to take for the base for comparison