Fig, shows a light and inextensible string passing over a smooth and weightless pulley. Two bodies of weights W_{1} and W_{2} are attached to the two ends of the string.

Let W_{1}>W_{2}, the weight W1 will move downwards, whereas smaller weight W_{2} will move upwards. For an inextensible string, the upward acceleration of the weight W_{2} will be equal to the downward acceleration of the weight W_{1}.

As the string is light and inextensible and passing over a smooth pulley, the tension of the string will be same on both sides of the pulley. Consider the Motion of weight W_{1}(Down motion) W_{1}-T = m_{1}.a ...(i)

Consider the Motion of weight W_{2} (up motion)

T-W_{2} = m_{2}.a ...(ii)

Solved both the equation for finding the value of Tension (T) or acceleration (a)