Answer:
75 km/h
Explanation:
Speed = Distance divided by Time.
The train went a distance of 300 km in 4 hours. Therefore, the speed of the train is: 300 divided by 4 = 75 km/h
A) 
The total energy of the system is equal to the maximum elastic potential energy, that is achieved when the displacement is equal to the amplitude (x=A):
(1)
where k is the spring constant.
The total energy, which is conserved, at any other point of the motion is the sum of elastic potential energy and kinetic energy:
(2)
where x is the displacement, m the mass, and v the speed.
We want to know the displacement x at which the elastic potential energy is 1/3 of the kinetic energy:

Using (2) we can rewrite this as

And using (1), we find

Substituting
into the last equation, we find the value of x:

B) 
In this case, the kinetic energy is 1/10 of the total energy:

Since we have

we can write

And so we find:

Answer:
a) 200A
b) 10.2V
c) 2.04kW
d)
I=80A
V=4.08V
P=0.326kW
Explanation:
Here we have a circuit of one power source and two resistors in series, the first question is asking for the current, so according to Ohm's Law:

Where R is the equivalent resistance of the resistors in series
![R=0.0510+0.0090=0.0600[ohm]](https://tex.z-dn.net/?f=R%3D0.0510%2B0.0090%3D0.0600%5Bohm%5D)

To calculate the voltage dropped by the motor we have to apply the voltage divider rule:

The power dissipated supplied to the motor is given by:

now solving adding a 0.0900 ohm resistor:



The force is opposite to the displacement