Answer:
a) 17.49 seconds
b) 13.12 seconds
c) 2.99 m/s²
Explanation:
a) Acceleration = a = 1.35 m/s²
Final velocity = v = 85 km/h = 
Initial velocity = u = 0
Equation of motion

Time taken to accelerate to top speed is 17.49 seconds.
b) Acceleration = a = -1.8 m/s²
Initial velocity = u = 23.61\ m/s
Final velocity = v = 0

Time taken to stop the train from top speed is 13.12 seconds
c) Initial velocity = u = 23.61 m/s
Time taken = t = 7.9 s
Final velocity = v = 0

Emergency acceleration is 2.99 m/s² (magnitude)
Answer:
magnitude of A − B = 15.81 km
Explanation:
Vector A points in the negative y-direction and has a magnitude of 5 km. Vector B points in the positive x-direction and has a magnitude of 15 km.
According to Cartesian coordinate system, the resultant will start either from tail of A and ends at head of B and vice-versa.
A(0,-5)
B(15,0)
A - B = (-15 i - 5 j )
Magnitude of the vector is given by
|A - B| = 
|A - B| = 
|A - B| = 15.81 km
Speed = (frequency)x(wavelength).
Frequency = 6 Hz
Wavelength = 9mm = 0.009m.
Speed = (6 Hz) x (0.009m) = 0.054 m/s or 54 mm/sec.