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Is it F=ma? I looked around online.
Specifically, its Ft-Fg=ma.
Tension is just the force within a string, rope, wire, etc... and the same exact formula for any other force can be used, F=ma.
So, suppose you have a banana tied to a very light string which is tied to a tree branch. The tension in the string is just the same as F=ma, and in this case the mass is the mass of the banana and the acceleration is g, about 9.8m/s^2.
This is a simplification model, however, as there will be a slightly higher tension in the string at the top than at the bottom since it is supporting the mass of all the string below it.
Suppose you have a mass supported by two strings? If you know the angles created between the strings and the horizontal or vertical, you can use trigonometric functions and find that the sum of the two forces in the upwards y-direction is equal to the force of the downwards y-direction due to the mass and gravity. Since the mass isn't moving up or down, the forces must be in equilibrium, so that's how we know the sum of the two tensions in the upwards direction is equal to the downwards pull of gravity.
I can explain with more in-depth examples if you're interested!