Question

So we did an experiment of a cart on a track connected by a string through a frictionless pulley. The apparatus is similar to the one here http://hyperphysics.phy-astr.gsu.edu/hbase/class/phscilab/frictionex.html. To account for frictional force, we had to hang counter weights on the string and measure using a program to assure that the velocities were constant no matter what the force applied.

However, one of the questions to answer is consider the mass added onto the string. Should this mass be included in energy conservation calculations? Can it be argued either way?

Answer 1

The link is not working, but from reading it I think that you will need to consider the extra masses if they form part of the system you are analysing. For example the kinetic energy would need the total mass of the moving system to say how much there is.

Answer 2

Sorry; you have to delete the period at the end of the link.

I figured that it had to be considered. The friction is no more and cannot subtract from the gravitational PE but the mass is still present. Yet I don't think it would make any difference to include or not as long as consistency was kept in the mass of the system being equal to everything but the added mass to counteract the friction. Is that sound logic?

Answer 3

Yes, in that page it even says:

"The "energy account" that we are considering consists of just the hanging mass and the block to which it is attached by the string. "