Physics Newtons Law problem?

Question

Superman must stop a 100 km/h train in 150 m to keep it from hitting a stalled car on the tracks. If the train's mass is 360,000 kg, how much force must he exert? Compare to the weight of the train.

I was thinking that i would find the acceleration with the Kinematics equation...

V^2 = Vo^2 + 2a(x-xo)

The acceleration i came up with was -33.3333333 km/hr.

I then took Newtons formula to calculate the force.

F = ma

F = (360,000 kg) x (-33.3333333 km/hr)

F = -12,000,000 N

I do not understand what i am doing wrong... the answer should be -930,000 N which is %25 the weight of the train.

Thanks.

Answer 1

the SI unite is in m but you used km

100km/hr => 27.78m/s

find acceleration
Vf^2 = 2ad + Vi^2
0^2 = 2(a)(150) + 27.78^2
-771.73 = 300a
a = -2.57m/s^2

F = ma
F = (360,000kg)(-2.57m/s^2)
F = -925,200N

Answer 2

convert km/hr to m/s --> u'll get 27.77 m/s

use v^2=v(start)^2 + 2ax---> 0=771.6 + 2 x a x 150
ull get : -771.6=300 x a --> a=2.57 m/s
Force=ma=360000 x 2.57 = 925925.925N
Weight of train is = 360000 x 9.8
divide the two and multiply by 100 and ull get : 26% (i didnt get 25%)

What you did wrong is..when u calculated force, u cannot use km/hr..u haf2 use m/s.
Hope this helps! =)

Answer 3

Just move the stalled car, it's lighter.
You might also try using metres/second instead of km/hr.