An object with twice the mass will experience half the acceleration of the first mass. This is explained through Newton's second law, which states that the net force on an object equals its mass times acceleration (i.e. Fnet = ma). Rearranging this equation to solve for a, we get a = Fnet/m, where acceleraton is inversely proportional to mass. Thus, if you double the mass (2m instead of m) you are essentially multiplying Fnet/m by 1/2 (i.e. [tex] \frac{Fnet}{2m} = ( \frac{1}{2}) \frac{Fnet}{m}[/tex]) and thus halving the acceleration.
