On the smooth horizontal plane, a car B with mass m = 2kg is at rest, and there is a linear track with length L = 1m on the upper surface of the car At present, a small block a with mass m = 1kg slides onto the left end of car B at speed v = 5m / s, and the dynamic friction coefficient between block a and the horizontal track of car B is 0.3? The main question is (2) the formula MV = mvm-mv1 ① 1 / 2mV ^ 2 = 1 / 2mvm ^ 2-1 / 2mV ^ 2-umgl ② these two formulas are listed by me, but in the answer, the formula ② is 1 / 2mV ^ 2-umgl = 1 / 2mvm ^ 2 + 1 / 2mV ^ 2, why? What's wrong with my formula?

On the smooth horizontal plane, a car B with mass m = 2kg is at rest, and there is a linear track with length L = 1m on the upper surface of the car At present, a small block a with mass m = 1kg slides onto the left end of car B at speed v = 5m / s, and the dynamic friction coefficient between block a and the horizontal track of car B is 0.3? The main question is (2) the formula MV = mvm-mv1 ① 1 / 2mV ^ 2 = 1 / 2mvm ^ 2-1 / 2mV ^ 2-umgl ② these two formulas are listed by me, but in the answer, the formula ② is 1 / 2mV ^ 2-umgl = 1 / 2mvm ^ 2 + 1 / 2mV ^ 2, why? What's wrong with my formula?

You are wrong. 1 / 2mV ^ 2 = 1 / 2mvm ^ 2-1 / 2mV ^ 2-umgl. It should be initial kinetic energy = car kinetic energy + kinetic energy of a + work done to overcome friction. This is the law of conservation of energy. 1 / 2mV ^ 2 = 1 / 2mvm ^ 2 + 1 / 2mV ^ 2 + umgl As long as this formula will not be wrong: initial energy