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. A diagram of the situation can be found below:

http://w3.rcnuwc.no/public/physics/test/ict%20website-2-/Images/Practicals%20images/prac2diag.gif

When the cart is released, and the mass is allowed to fall, the force of the string of the cart on the cart slightly decreases.What is causing this to happen?

Question

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. A diagram of the situation can be found below:

http://w3.rcnuwc.no/public/physics/test/ict%20website-2-/Images/Practicals%20images/prac2diag.gif

When the cart is released, and the mass is allowed to fall,  the force of the string of the cart on the cart slightly decreases.What is causing this to happen?

anonymous · Answer

Here is a Force vs. Time graph of the cart. As you can see, the tension in the string slightly decreases immediately after the cart is released. 
@ganeshie8, @Preetha, and @Astrophysics, I have re-worded the problem. This might make it easier to answer. Anyone is welcome to help with any information. Thank you very much.   :D

brrandyn · Answer

Presumably, the following model I've drawn is a representation of your given scenario: |dw:1448334093669:dw|

The "force of the string of the cart," otherwise, force tension, decreases because the hand is no longer acting for the cart to be in equilibrium. The force of gravity on the mass causes the force tension to decrease, therefore, pushing the cart forward.