A block of mass $M_{1}=0.640 \mathrm{~kg}$ is initially at rest on a cart of mass $M_{2}=0.320 \mathrm{~kg}$ with the cart initially at rest on a level air track. The coefficient of static friction between the block and the cart is $\mu_{s}=0.620$, but there is essentially no friction between the air track and the cart. The cart is accelerated by a force of magnitude $F$ parallel to the air track. Find the maximum value of $F$ that allows the block to accelerate with the cart, without sliding on top of the cart.