Superman: Stop a Train in Minimum Time

[BuBbLeS01]

Homework Statement

A.) Find the speed at which Superman (mass=79.0 kg) must fly into a train (mass = 17026 kg) traveling at 70.0 km/hr to stop it.
B.) Running into the train at that speed would severely damage both train and passengers. Calculate the minimum time Superman must take to stop the train, if the passengers experience an average horizontal force of 0.500 their own weight.
C.) How far does the train then travel while being slowed to a stop?

So I put the train traveling to right, positive x-direction and superman traveling left, negative x-direction

Homework Equations

Pf-Pi = 0
Pf = Pi

Ms= 79kg
Mt = 17026kg
Vti = 70km/hr = 19.444m/s
Vft = 0m/s (because it is coming to a stop)
Vfs = 0m/s (because it is coming to a stop)
Vsi = ?

The Attempt at a Solution

Pf = Pi
(Ms * Vfs) + (Mt * Vtf) = (Ms * Vsi) + (Mt *Vti)
0 = (Ms * Vsi) + (Mt *Vti)
(-Mt * Vti) / Ms = -Vsi (Negative because he is moving to the left

I am not getting the right answer, can someone please help me out?

 

[mgb_phys]

First part is conservation of momentum (=mv)
Second is just v = u + a t with a = 0.5g

 

[BuBbLeS01]

Isn't that what I did...for the first part?

 

[BuBbLeS01]

So for the first part isn't it...
Pf = Pi
(Ms * Vfs) + (Mt * Vtf) = (Ms * Vsi) + (Mt *Vti)

 

[BuBbLeS01]

I guess not because its saying incorrect.

 

[BuBbLeS01]

I need help with the second part...but what is u??

 

[BuBbLeS01]

Im losing my mind with part B and C...does anyone know how to do these?

 

[mgb_phys]

a, speed = 70km/h x (17026/79) =15086 km/h

b, v=u +at v=0, a=-0.5g = -4.9 m/s^2 u=19.4m/s
t = 3.96s

c, v^2 = u^2 + 2as as above, 19.4^2 = 2 *0.5 *g *s
s = 19.4^2/9.8 = 38.6m

 

[BuBbLeS01]

Thank you!