The driver provides a constant force on the engine through the foot pedal. Eventually the van stops accelerating and reaches a constant speed. c Explain why the van reaches a constant speed if the driver provides a constant driving force to the van.

It follows from Newton's second law that there is some counteractive force that cancels out the force exerted by the engine - it's most likely drag due to air resistance in combination with static friction between the tires and the road. The car is moving at constant speed past a certain point, so the net force on the car is

∑ F = (force from engine) - (resistive forces) = 0

v=ir = 3x15 =45 v

electric field is a function of 1/r^2

yes, the force per unit charge for a point charge is inverse square law.