A 1000kg car exerts a horizontal force of 3500N to pull a 450kg trailer, and the frictional force for the trailer, calculated using a coefficient of 0.15, must be considered in determining the force exerted on the trailer. The frictional force is derived from the equation (0.15)(450)(9.81), which needs to be either added to or subtracted from the car's force. The discussion also shifts to a separate problem involving a car's skid mark of 88m on a rainy day with a friction coefficient of 0.42, prompting a calculation of the car's speed at the moment brakes were applied. It is noted that the car's mass is irrelevant in this context due to the cancellation of mass in the equations used. Understanding these principles is crucial for solving both problems effectively.