For accelerating dynamic trolleys.With the length of elastic cords with brass eye let at each end. In use cord is attached with the trolley by slipping one of the eye lets over the front dowel peg. ( One or more cords may be used simultaneously depending upon the acceleration required) Constant acceleration will arise if the cord is stretched a constant amount, say, to length of the trolley. The cords are packed in tens mounted on a card fore easy storage.