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 the floor is μMg where μ is the coefficient of static friction. My load will slide when F1 is greater than F2. So the load starts to slide when F1=F2 or MA=μMg. In other words after the M's cancel the acceleration at which my load starts to slide is independent of it's mass. All loads will slide when the van has an acceleration greater than μg. But I know that light things slide all over the place whereas 2 tonne masses of iron tend to stay put. So what is wrong with my logic?