To solve this problem it is necessary to use the concepts related to the Hall Effect and Drift velocity, that is, at the speed that an electron reaches due to a magnetic field.
The drift velocity is given by the equation:
Where
I = current
n = Number of free electrons
A = Cross-Section Area
q = charge of proton
Our values are given by,
The hall voltage is given by
Where
B= Magnetic field
n = number of free electrons
d = distance
e = charge of electron
Then using the formula and replacing,
The force exerted on the car during this stop is 6975N
<u>Explanation:</u>
Given-
Mass, m = 930kg
Speed, s = 56km/hr = 56 X 5/18 m/s = 15m/s
Time, t = 2s
Force, F = ?
F = m X a
F = m X s/t
F = 930 X 15/2
F = 6975N
Therefore, the force exerted on the car during this stop is 6975N
By the Pythagorean theorem we get 
. Thus the resultant velocity will be 5 m/s.
However there is a typo in the choices. It should take 25 seconds to cross the river, not 20 seconds, since the boat actually travels 125 meters due to the water current, and 125/5 = 25 seconds.
Alternatively you can calculate the eastward component of travel and get 100 / 4 = 25 seconds.
