Originally Posted by

**topsquark** I agree with your teacher. The impluse-momentum theorem says:

$\displaystyle I = F_{ave} \Delta t = \Delta p$

Now, since both the retarding force of N and the braking forces are constant, we may simply take $\displaystyle F_{ave} = f + \, N$ (where f is the force due to the brakes) as a constant. Thus:

$\displaystyle -(100000 + f) \Delta t = m(v - v_0) = -m v_0$ (The final speed of the train is v = 0 m/s.)

So we have:

$\displaystyle -(100000 + f) \cdot 40 = - \cdot 25$

$\displaystyle 100000 + f = 5000 \cdot 25 =$

$\displaystyle f = = = 25000 \, N$

-Dan