Tension and acceleration in a string:

Consider a block supported by a string. The upper end of the string is fixed on a stand. Let w be the weight of the block. The block pulls the string downwards by its weight. This causes a tension T in the string. The tension T in the string is acting upwards at the block. As the block is at rest, therefore, the weight of the block acting downwards must be balanced by the upwards tension T in the string. Thus, the tension T in the string must be equal and opposite to the weight w of the block.