Tension -- how is mechanical tension even defined? Hello, I'm currently a student in AP Physics having a little trouble with the concept of tension, especially that of a pulley system or an Atwood machine. First, how is tension even defined? Is it the reaction force to the applied force on the string? I keep getting confused because I have this impression that it is a pair force, but not sure which is defined as tension in the string. For ex, for an Atwood machine, is tension the reaction force of the applied gravitational force due to the weights on each end? And also, I'm having some trouble generally understanding the concept that the tensions of the strings on each side of the pulley may be different as well. I have a vague understanding of how the resultant final acceleration of the system may be calculated, but again, am failing to visualize it conceptually. If you could take your time to explain or point me towards the source, it would be greatly appreciated. Thanks!