college physics
posted by john .
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?

(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. 
for part b, what would be the percentage. i got 10% but it isnt right

John: show your work, you wouldn't want anyone to think you are answer grazing.
percent= force/mass*g * 100 
i got it its 79% i was label some of the numbers with the wrong variables. tahnks and will keep that in mind