12. a 15,000 kg train car is rolling at a speed of 3.2 m/s. compute the time required for a force
of 1200 n to stop the car.

40 s

Explanation:

By Newton's 2nd law,

The rate of change of momentum is directly proportional to the unbalance force applied on the object,

By that you can get the equation,

F = ma ⇒ a = F/m

but  a = (v-u)/t

So we get, t = (v-u)×m/F

where terms are in usual meaning

t =(3.2-0)×15000/1200  = 40 s

96.77 amp

explanation:

rl = 250/100 = 2.5 ohms.

egl = 250 + (100 × 0.1) + 2 = 262 v.

at 700 r.p.m., eg2 = 262 × 700/1000 = 183.4 v.

if ia is the new current, then eg2 - 2 - (ia × 0.1) = 2.5 ia

which means this will give ia = 96.77 amp.