I recently did a problem where two masses were connected to the same massless string in an Atwood's machine, yet they had different accelerations. This wasn't really intuitive to me. If one mass had a greater magnitude of acceleration than the other mass, then it would imply that the bit of string above the one mass also had a greater acceleration than a bit above the other mass. If tension is the same throughout and the tiny bits of string are massless, then where does the extra force come from to accelerate the one bit of string more than the other? I know my thinking is wrong, but I'm trying to find out where. I think it might lie in my hazy understanding of tension. Also, I realize my question might be hazy as well, I will try to post a diagram of what I am asking later when I have time.