I am attaching two pics one is solved problem (17) and other isthe solution of a problem where power of engine is to be calculated. The question is what is the power of engine which can just pull a train of mass 5000quintals up an incline of 1 in 50 @ 54 km/hr. The resistance due to friction is 0.8N/quintal. Take g=9.8.
My question is why mg is being multiplied by sin theta at one place and cos theta is being used in the other.
Please refer 17 & 39(a) in the pic.