f' = frequency observed by the police car after sound reflected from the vehicle and comes back to police car = 1250 Hz
f = frequency emitted by the police car = 1200 Hz
V = speed of sound = 340 m/s
v = speed of vehicle = ?
frequency observed by the police car is given as
f' = f (V + v)/(V - v)
inserting the values in the above equation
1250 = 1200 (340 + v)/(340 - v)
v = 6.9 m/s
So obviously the 20N pulls more than the 5N. Friction opposes direction of motion, so the 5N opposes the 20N and the two forces are against each other.
Net force would be in the direction of the 20N force: 20N - 5N = 15N.
Force = mass*acceleration
15 = (5)*acceleration
acceleration = 3m/s^2
We would have to use wind or another source
We can solve the problem by using the first law of thermodynamics:

where
is the variation of internal energy of the system
Q is the heat added to the system
W is the work done by the system
In this problem, the variation of internal energy of the system is

While the heat added to the system is

therefore, the work done by the system is
