# Homework Help: Just a small doubt

1. Nov 16, 2005

### vaishakh

When two objects are tied to some two ends of a string on a pulley hung on a surface, the normal force applied by the surface on the pulley is twice the tension of the string. Why is this so?
Infact one of the questions had such a pulley hung on a pulley with the help of a string whose other end is tied to a object. The question was to find the acceleration of all the three objects and I found the relative (to pulley) acceleration of first two objects. Now this pulley was given to be massless. So I thought the mass on this side should be taken as the sum of the two masses tied while finding the acceleration of pulley. This resulted in approaching a wrong answer. However when the mass was taken to be 2T/g where t is the tension on the string and g is the acceleration due to gravity my answer was perfect. I didn't understand the concept behind this. I mean what about those masses

2. Nov 17, 2005

### dx

The strings are pulling the pulley down with a force with contributions from each string. But since the pulley is not accelerating downward, there must be a compensating force of the same magnitude and opposite direction coming from the place where it is hung.

Last edited: Nov 17, 2005