A car cruises down the highway at constant velocity 50 mph. The backwards force of wind resistance and friction have a combined strength of 5000 newtons. The car’s engine causes a forward force to be exerted on the car. Is this forward force less than 5000 newtons, equal to 5000 newtons, or greater than 5000 newtons?

According to the second Newton law, the net force that acts over a body is equal to its mass multiplied by its acceleration. In this case, we know that there is a force of 5000 N which is contrary to the movement, and which is overcome by a force that the engine causes. We also know that there is no acceleration, because the velocity is a constant (the acceleration is the change of velocity through the time).

The net force [tex]F_N[/tex] over the car is:

[tex]F_N=F_{engine}-F_{RyF}[/tex]

Where N subscript means net, and RyF subscript means the resistance and friction; for the second law we have:

[tex]F_N=m*a[/tex]

But, [tex]a=0[/tex], so: [tex]F_{N}=0[/tex]

[tex]F_{engine}=F_{RyF}=5000N[/tex]

Always you have a body without acceleration (or constant velocity, even zero), the net force acting on it is zero because the Newton second law says us that if there were a force, there would be acceleration.