3a

It takes work to shake a can of juice. What happens to the work?

It goes primarily into increasing the temperature of the juice, by increasing the average kinetic energy of the molecules in the juice

3b

Many heating systems use water as the medium (some radiators, for example). The specific heat of water is relatively large. Why does this make water a good medium for such systems?

Q = cmD T

Because the specific heat of water is quite large, it can contain a relatively large amount of thermal energy per unit mass with a relatively small increase in temperature.  Since the water is a liquid, it is relatively easy to transport from one location to another, and so large quantities of energy can be moved from one place to another with relative simplicity using water.