A traveling wave has a frequency of 7.31 x 1017 GHz. Determine how long it would take a two successive crest to pass a given point.

The forces acting on the pen which is still on the table can have two forces acting on them.  The forces are gravitational force and the equal and opposite force to the gravitational forces.

The equal and opposite forces that is applied on the pen keeps the pen still on the table.

So, the statement that no force is applied on the pen which is kept still on the table is wrong as two forces are applied on the pen.

As both the forces are equal and opposite so it is cancelled and is still.