What factors should be considered when calculating motor size for a trolley?

[fakhar]

I want to build a trolley with four wheels and towed to a rope having pulley on both ends connected to a electric motor. one of the pulley will be drive and other will be idle. what will be the proper procedure to calculate the motor size for this type of problem. thanks in advance

 

[CWatters]

First you will need to know the forces required to move and or accelerate the trolley. You could break the total force down into three parts...

1) Force required to climb slope if any.
2) Force required to overcome the rolling resistance (Depends on smooth or rough surface? Type of tyres, pressure etc)
3) Force required to accelerate from rest to max speed. Depends on mass and acceleration required.

Some things can be calculated (such as 1 and 3) but 2 may have to be measured or estimated.