Which one among the following is the correct focal length of a combination of lenses of power 2.5 D and -2.0 D?

The AC mains domestic supply current in India changes direction in every 

A block of mass 20 kg slides on a rough horizontal plane surface. Let the speed of the block at a particular instant is 10 m/s. It comes to rest after travelling a distance of 20 m. Which one of the following could be the magnitude of the frictional force?

Given below are the four cases in which certain heat transfer is taking place:

1. Ice is melting in a glass full of water

2. Water is boiling in an open container

3. A metal rod is heated in a furnace

4. A cup of coffee is allowed to cool on a table

In which of the above cases, the Newton's Law of Cooling is applicable ?