6. (15 points) A tank with a mass of $m_{1}=3$ kg rests on a cart with a mass of $m_{2}=15$ kg, which is accelerated with an acceleration of $a=4 \mathrm{~m} / \mathrm{c}^{2}$. The coefficient of friction between the tank and the cart is $\mu=0.6$. Determine the frictional force acting on the tank from the cart.
Answer: $12 H$

Solution. Under the given conditions, we are talking about the force of static friction. According to Newton's second law: $F_{n p}=m_{1} a=12 H$.