Hello!
<em>a) What is the change in momentum</em>?
The change of the momentum is the velocity, because the velocity is reduced to zero.
We can calculate the aceleration, applicating the formula:
Like you see, the final velocity will be zero, so:
Like you see, the aceleration applicate by superman is of <u>-10 m/s^2.</u>
If the aceleration is negattive, means the velocity will decrease.
<em>b.) What is the magnitud of the force wich superman exerts on the train?</em>
For calculate the force applicate for superman, lets applicate the second law of Newton:
Lets replace and resolve it:
F = 8x10^3 kg * (-10 m/s^2)
F = 8000 kg * (-10 m/s^2)
F = -80 000 N
The force applicated is of <u>-80000 Newtons.</u>
When the force is negative means the force applicated in the another direction of the object what is traveling.