A 075 lbm object traveling at 200 fts enters a viscous fluid

A 0.75 lbm object traveling at 200 ft/s enters a viscous fluid and is essentially brought to rest before it strikes the bottom/side. What is the increase in internal energy considering the liquid and the object as the system? You can neglect the potential energy change.

Solution

As we are considering both liquid and object as a system there will be no change in internal energy of the system because internal energy can only be changed by transfer of matter or by transfer of heat and work to or from the system.

In this case the loss in kinetic energy of object is converted to equal amount of other energy which will store in liquid and as a whole the net change in internal energy of the system (object+ liquid) is zero.

So there is no increase in internal energy of the system.

If we would have considered only liquid as system then the internal energy of liquid will increase by equal amount of loss in kinetic energy of object.