I need to buy a DC motor for an electric skateboard project. I need to calculate the speed at which it can travel with a person on top. I can calculate the speed at which the skateboard can go but is does not account for the weight of a person as that will surely decrease the speed. What formula can be used to calculate the speed with for say a 120kg person standing on the skateboard. The skateboard can go at a speed of 21mph how much will it decrease due to a person's weight? Any known formulas? Also, what formula can I use to calculate the speed on an incline? Please help me out.