Conservation of mechanical energy

Click Attachments & Additional Options... while creating a post, and upload the image. I think I understand what's happening though.

You are transforming the potential energy of the 1kg box 1m above the ground to kinetic energy on a 4kg mass system.

Ep = 1kg 1m 9.8m/s2 = 9.8 J

This is the kinetic energy for the total system with 2 boxes (4kg).

For the 1 kg box only, kinetic energy is 1/4 of total kinetic energy (b/c its mass is 1/4 of the total mass of the system and energy is proportional to mass).

So, kinetic energy for the 1 kg box is 9.8 J / 4 = 2.45 J.

Remember to use W - T = ma, which would yield an acceleration of half the value he calculated and a final kinetic energy.