Thanks for that WhyIsItSo.
I agree, but also the heavier the load the greater the force needed to accelerate it. If my original maths is correct then the masses cancel and the acceleration at which the load slides is independent of its mass.
Similar problem with F1 (or Indy etc.) cars...
I drive a van. When I accelerate the load inside my van of mass M must be accelerated at acceleration A which is the same as the acceleration of my van by a force F1, where F1=MA.
The mass M has a reactive force Mg on the floor of the van and so the force, F2 needed to make it slide is over...