This is not a homework question, but simply a custom problem from class. I've attempted to answer it but realized I was incorrect, and can't think of an alternative answer. 1. The problem statement, all variables and given/known data A cart is sitting on a frictionless track. A string is attached to this cart, which runs over a pulley with a mass tied on the other end. A hand is holding the cart in place. When the cart is let go, and the mass allowed to fall, the force of the string on the cart slightly decreases. What is causing this to happen? 2. Relevant equations [tex]f=ma[/tex] 3. The attempt at a solution Initially, I had thought that the force of the hand on the cart was causing additional tension in the string, but this could not be true. All the hand was doing was creating an equal and opposite force on the cart, causing it to remain at rest. The tension in the string should not have been affected by the hand. However, I can't think of any other explanation as to why this would occur. Any ideas / hints?