Write a grammar for the language consisting of strings that have n copies of letter 'a' followed by the same number of copies of letter 'b', where n > 0. for example, the string ab, aaabbb are in the language but abb, or bbbaa are not.

only #2 represents an object at rest on a table.

in #1, there is no normal force due to the table; the object is accelerating downward.

in #3, the vertical forces are not balanced; the object is accelerating downward.

in #4, neither the vertical nor the horizontal forces are balanced. the object is accelerating upwards and to the right.