just a little pyhsics help?

Question

1.A freight train has a mass of 2 107 kg. If the locomotive can exert a constant pull of 9 105 N, how long would it take to increase the speed of the train from rest to 85 km/h?
s


2.A 50 kg person escapes from a burning building by jumping from a window 50 m above a catching net. Assuming that air resistance exerts a 95 N force on the person during the fall, determine the person's velocity just before hitting the net.
m/s

Answer 1

1) F= m*@ so @= F/m= 9105/2107 m/s^2
v= @t so t= v/@= 85,000/3600*2107/9105 =5.46 s
2)Net Force =mg-air resistence = 50*9.8 -95=395 N
395 =m*@ so @= 395/50= 7.9 m/s^2
50 = 1/2 *7.9 *t^2 so t = sqrt( 100/7.9)= 3.56s
v= @t = 7.9*3.56 = 28.11m/s

Answer 2

♠ Newton’s 2nd law says f=ma, hence acceleration a=f/m;
the train gains speed v=a*t, hence time
t=v/a = vm/f =(85*1000/3600) *2107 /9105 =5.46 s;
♣ Newton’s 2nd law says f=ma,
where f=f1-f2, f1=mg, f2=95N, m=50kg, hence a=(f1-f2)/m;
h=0.5a*t^2, where h=50m, hence time in fall t=√(2h/a);
v=a*t = a√(2h/a) = √(2ha) =√(2h*(f1-f2)/m) =
=√(2*50 *(50*9.8 -95)/50) =28.1 m/s;