If you add mass to a car on a ramp the acceleration of the car does not increase proportional to the increase in mass. why not?

For the same reason that a penny and a school bus fall with the

same acceleration if you drop them together from a high building.

It's true that the bus has, let's say, a million times the mass of the penny,

so the gravitational force on it is a million times as strong as the force on

the penny. BUT ... remember that since F=mA, A=F/m . Do you see that ?

Acceleration is inversely proportional to mass ! So when you have one mass

that's a million times the size of another one, the force of gravity on it is

a million times as strong as the force on the little one, but that's exactly

the force it takes to accelerate the big one as much as the little one.

Big mass INcreases the force of gravity and DEcreases the acceleration,

by exactly the same factor ! So when gravity is the only force on the

object, every object accelerates at exactly the same rate, no matter

what its mass is.