1. The problem statement, all variables and given/known data Two particle of mass m each are tied at the ends of a light string of length 2a. The whole system is kept on a frictionless horizontal surface with the string held tight so that each mass is at a distance ‘a’ from the center P (as shown in the figure). Now, the mid-point of the string is pulled vertically upwards with a small but constant force F. As a result, the particles move towards each other on the surface. The magnitude of acceleration, when the separation between them becomes 2x is 2. Relevant equations 3. The attempt at a solution http://2.bp.blogspot.com/_lRKubY9oOyI/RrGGAxqjRUI/AAAAAAAAAHQ/TtdYi47lpDc/s1600-h/Newton 2Tsinθ = F T = F/2sinθ where sinθ = (a2 - x2)1/2 / a Net force on each particle is T. So T = maacceleration So, aacceleration = F/2msinθ However, the answer is F/2mtanθ. I don't get why. Please help?