Consider an example of a mass hanging by a string. The string is fixed at the other end. The mass will be at rest because weight of the object is balanced by tension in the string upwards.Consider another example in which the other end of the string is not fixed. Now if that object is coming down with some acceleration meaing it has some net force acting on it. Why is that so when weight of the object has equal reactional tension..So should'nt it be balanced by equal upward and downward forces??