As shown in the figure, Mn and PQ are two metal rails horizontally placed in parallel with each other, and the magnetic induction line with uniform magnetic field is vertical As shown in the figure, Mn and PQ are two metal rails horizontally placed parallel to each other. The magnetic induction line of uniform magnetic field is perpendicular to the plane of the guide rail. The left end of the guide rail is connected with a resistance of resistance value. A voltmeter is connected at both ends of the resistance. The vertical guide rail is connected across a metal rod ab. the mass, resistance and dynamic friction coefficient between AB and the guide rail are μ = 0.5, and the resistance of the guide rail is ignored. Now the constant force of F = 0.7n is used to pull AB horizontally to the right, Let it start to move from rest. After time, AB starts to move at a constant speed. At this time, the voltage expression u = 0.3V, the acceleration of gravity. Find (1) the power of external force F when AB moves at a constant speed. (2) the electric quantity passing through R during the acceleration of AB rod Mainly ask the second question, I searched, there is a "impulse theorem" to do the second question, but we did not learn. Is there any other way to ask the second question?

As shown in the figure, Mn and PQ are two metal rails horizontally placed in parallel with each other, and the magnetic induction line with uniform magnetic field is vertical As shown in the figure, Mn and PQ are two metal rails horizontally placed parallel to each other. The magnetic induction line of uniform magnetic field is perpendicular to the plane of the guide rail. The left end of the guide rail is connected with a resistance of resistance value. A voltmeter is connected at both ends of the resistance. The vertical guide rail is connected across a metal rod ab. the mass, resistance and dynamic friction coefficient between AB and the guide rail are μ = 0.5, and the resistance of the guide rail is ignored. Now the constant force of F = 0.7n is used to pull AB horizontally to the right, Let it start to move from rest. After time, AB starts to move at a constant speed. At this time, the voltage expression u = 0.3V, the acceleration of gravity. Find (1) the power of external force F when AB moves at a constant speed. (2) the electric quantity passing through R during the acceleration of AB rod Mainly ask the second question, I searched, there is a "impulse theorem" to do the second question, but we did not learn. Is there any other way to ask the second question?

Let the average current be I, then the average acceleration a = (F - μ mg bil) / m
V=at=（F-μmg-BIL）t/m=（Ft-μmgt-BQL）/m