Re: Need soln explaination

["Virendra K. Saxena" <saxena@physics.purdue.edu>]

> I got a problem out of an older physics text (one without a solutions
> manual). My students and spent a while debating the method of attaining
> the solution. Can someone explain how they would solve it -using energy.
>
> PROBLEM
> Masses of 3 kg and 2 kg are hanging at opposite ends of a cord that
> passes over a frictionless pulley. The system is held stationary for a
> while, then released. What will be the speed of the masses when the 3 kg
> mass has descended 0.5 m below its starting point? (USE ENERGY
> RELATIONSHIPS TO SOLVE.)
>
> -tony

    Change in gravitational potential energy of the system

      = -3gh+2gh = -gh   (3 kg mass loses PE and 2 kg mass gains PE)

     Change in KE = (1/2)3 v^2 + (1/2)2 v^2 = 5v^2/2

    Conservation of energy gives
    
     5v^2/2 - gh = 0

    Solve this using h=0.5 m and you get v=1.4m/s.